MBBS First Year Viva Questions - Endocrine Function Tests

Tutorial Batch | University Level | Medium to Hard | Interlinked

PART A: THYROID GLAND FUNCTION TESTS

40 Viva Questions

Model Answer: Serum TSH (Thyroid Stimulating Hormone) is the best first-line screening test. The pituitary is exquisitely sensitive to free T4 - a small change in free T4 causes a large logarithmic shift in TSH. The ultrasensitive immunometric TSH assay detects changes before clinical symptoms appear, making it the most sensitive and specific test for both hyper- and hypothyroidism. Normal range: 0.5-5 μU/mL. (Schwartz's Principles of Surgery, 11e)

Model Answer: Elevated thyroid-binding globulin (TBG) - as seen in pregnancy, estrogen/OCP use, or congenital TBG excess. Total T4 measures both free and protein-bound hormone. Since biologic activity depends only on the free fraction, this patient is euthyroid if free T4 is normal. Confirmation: measure free T4 (reference range 12-28 pmol/L), which will be normal.

Model Answer: TSH secretion by the anterior pituitary follows a log-linear inverse relationship with free T4 - a 2-fold change in free T4 results in roughly a 100-fold change in TSH. This amplification makes TSH an early warning signal. A patient can still have free T4 within the "normal" range but show a clearly abnormal TSH, identifying subclinical thyroid disease before the patient becomes symptomatic.

Model Answer: Radiolabeled T3 is added to the patient's serum along with an ion-exchange resin. If free T4 is elevated, most TBG binding sites are already occupied by endogenous T4, leaving fewer sites for the labeled T3 - so more labeled T3 binds the resin (high resin uptake = high free T4). Conversely, in hypothyroidism or elevated TBG, more labeled T3 binds TBG (low resin uptake). This is an indirect index of TBG saturation.

Model Answer: Trust free T4. Total T4 is artifactually elevated by high TBG even in a euthyroid state. Free T4 measures only the biologically active, unbound fraction. In this case, if free T4 is also elevated alongside a suppressed TSH, the patient likely has gestational hyperthyroidism. The TBG elevation makes total T4 unreliable; free T4 and TSH together give the accurate picture.

Model Answer: Two monoclonal antibodies are used - one is bound to a solid matrix and captures serum TSH; the second binds a different epitope on the same TSH molecule and carries a signal label (radioactive, enzymatic, or fluorescent). Signal is proportional to TSH concentration. Sensitivity comes from the two-antibody "sandwich" that eliminates cross-reactivity with structurally similar glycoproteins (LH, FSH, hCG), achieving detection limits of ~0.01-0.02 μU/mL. (Schwartz's Principles of Surgery, 11e)

Model Answer: Total T3 is specifically indicated in T3 thyrotoxicosis - a condition where a patient is clinically hyperthyroid with suppressed TSH, but total T4 and free T4 are normal. T3 levels are also elevated early in hyperthyroidism and can rise before T4. Note: total T3 is not a useful screening test for hypothyroidism because T3 is maintained near normal even in mild-to-moderate hypothyroidism due to compensatory deiodination of T4.

Model Answer: T3 thyrotoxicosis occurs when hyperfunctioning thyroid tissue preferentially secretes T3 over T4. Biochemical basis: T3 is 3-5x more potent than T4 and has faster onset. Causes include: toxic adenoma, toxic multinodular goiter, early Graves' disease, and iodine-deficiency goiter. Labs: suppressed TSH, normal T4, elevated T3. Identifying it is clinically important because standard T4-based testing will miss it.

Model Answer: 500 μg TRH is given IV; TSH is measured at baseline, 30, and 60 minutes. Normal response: TSH rises by at least 6 μIU/mL from baseline.

• Flat response (no rise): indicates primary hyperthyroidism or pituitary TSH deficiency (secondary hypothyroidism)
• Exaggerated/prolonged rise: suggests primary hypothyroidism (chronically elevated endogenous TRH = sensitized pituitary)
• This test has been largely replaced by ultrasensitive TSH assays. (Schwartz's Principles of Surgery, 11e)

Model Answer: Refetoff's syndrome (Resistance to Thyroid Hormone, RTH) is caused by mutations in the thyroid hormone receptor beta (TRβ) gene. Target tissues are resistant to T4 action. The pituitary compensates by secreting more TSH, driving T4 production. Result: elevated free T4 and total T4 with a paradoxically normal or elevated TSH - the opposite of what you'd expect in hyperthyroidism. This can mislead clinicians into misdiagnosing central hyperthyroidism.

Model Answer: Anti-TPO (anti-microsomal) antibodies are more sensitive - present in ~95% of Hashimoto's patients vs. ~80% for anti-Tg. Anti-TPO indicates active autoimmune damage to thyroid follicular cells. Anti-Tg reflects immune response against stored thyroid hormone. Neither determines thyroid function directly - they confirm autoimmune etiology. Both can be elevated in Graves' disease, multinodular goiter, and occasionally thyroid carcinoma. (Schwartz's Principles of Surgery, 11e)

Model Answer: After total thyroidectomy + RAI ablation, serum thyroglobulin (Tg) is the tumor marker of choice for recurrence. Undetectable Tg = no residual or recurrent differentiated thyroid tissue - reassuring. However, anti-Tg antibodies can interfere with the Tg assay (immunometric interference), giving falsely low or undetectable readings even when disease is present. Therefore, anti-Tg antibodies must always be measured alongside Tg; a rising anti-Tg titer may itself signal recurrence. (Schwartz's Principles of Surgery, 11e)

Model Answer: OCP estrogen stimulates hepatic TBG synthesis, increasing circulating TBG. This leads to:

• Total T4 elevated (more bound T4)
• T3 resin uptake decreased (more TBG binding sites available, less labeled T3 goes to resin)
• Free T4 normal (actual biologic activity unchanged)
• TSH normal (pituitary sees normal free T4) The patient is euthyroid. The free T4 index (Total T4 × T3RU) can correct for TBG changes. (Berek & Novak's Gynecology)

Model Answer: FTI = Total T4 × T3 Resin Uptake. It mathematically corrects total T4 for TBG levels. When TBG is high (pregnancy, estrogens): Total T4 is high but T3RU is low - FTI stays normal. When TBG is low (nephrotic syndrome, androgens): Total T4 is low but T3RU is high - FTI stays normal. FTI approximates free T4 without needing a direct free T4 assay, though direct free T4 measurement is now preferred.

Model Answer: Nephrotic syndrome causes massive urinary protein loss, including TBG. With less TBG circulating, less T4 is bound, so total T4 is low. However, the free T4 (biologically active fraction) remains normal because the pituitary's TSH feedback loop adjusts to maintain physiologic levels of free hormone. TSH and free T4 will both be normal in a euthyroid nephrotic patient. This is an important cause of euthyroid sick/low-T4 syndrome to distinguish from true hypothyroidism.

Model Answer:

• ¹²³I: Low-dose radiation, half-life 12-14 hours. Used for diagnostic imaging of lingual thyroid, goiter location, and nodule characterization.
• ¹³¹I: High-dose radiation, half-life 8-10 days. Used for treatment of hyperthyroidism and screening/treatment of differentiated thyroid cancer metastases.
• ⁹⁹ᵐTc pertechnetate: Taken up (but not organified) by thyroid; shorter half-life, less radiation. Increasingly used for routine thyroid evaluation. Sensitive for nodal metastases. (Schwartz's Principles of Surgery, 11e)

Model Answer: "Hot" nodule: traps more radioactivity than surrounding tissue - indicates autonomous function. Malignancy risk <5% because well-differentiated cancers rarely have autonomous iodine uptake. "Cold" nodule: traps less radioactivity - non-functional area. Malignancy risk ~20% because malignant cells often lose the ability to concentrate iodine. However, most cold nodules (80%) are benign cysts, colloid nodules, or thyroiditis. FNA cytology is required to characterize cold nodules. (Schwartz's Principles of Surgery, 11e)

Model Answer: ¹⁸F-FDG PET (combined with CT) is used to detect metastatic differentiated thyroid cancer when RAI scan is negative but Tg is elevated - this is the "flip-flop phenomenon." Dedifferentiated thyroid cancer cells lose iodine uptake (RAI-negative) but increase glucose metabolism (PET-positive). The limitation: normal thyroid tissue also has high glucose metabolism, making PET poor for evaluating primary nodules. Reported malignancy rates in incidentally found PET-positive thyroid lesions range 14-63%. (Schwartz's Principles of Surgery, 11e)

Model Answer: Subclinical hypothyroidism is defined as an elevated TSH (>5 μU/mL) with a normal free T4 and no or minimal symptoms. The pituitary senses a slight fall in free T4 and compensates with more TSH, maintaining normal peripheral hormone levels. Diagnosis is purely biochemical. Important to repeat TSH after 3 months as transient TSH elevation occurs in non-thyroidal illness. Anti-TPO antibody positivity identifies patients at higher risk of progression to overt hypothyroidism (~5%/year).

Model Answer: Subclinical hyperthyroidism: TSH suppressed (<0.1 μU/mL) but free T4 and free T3 are within normal range, and symptoms are minimal. Overt hyperthyroidism: TSH suppressed + free T4 and/or free T3 are elevated + clinical symptoms. The distinction matters because subclinical hyperthyroidism increases risk of atrial fibrillation (3x) and osteoporosis, warranting treatment in specific populations even without overt symptoms.

Model Answer: Calcitonin (normal basal: 0-4 pg/mL) is secreted by C-cells (parafollicular cells) derived from neural crest. It is a sensitive and specific tumor marker for medullary thyroid carcinoma (MTC), which arises from C-cells. It is used for: (1) diagnosis of MTC, (2) screening family members in MEN-2A/2B syndromes, (3) post-operative monitoring for recurrence. Elevated calcitonin in a thyroid nodule patient should prompt calcitonin-stimulation testing (pentagastrin or calcium provocation). (Schwartz's Principles of Surgery, 11e)

Model Answer: In early and mild hypothyroidism, T3 levels remain normal or near normal due to: (1) upregulation of type-2 deiodinase in peripheral tissues, which preferentially converts T4 → T3; (2) elevated TSH stimulating preferential secretion of T3 from the thyroid gland. T3 therefore falls late in the course of hypothyroidism. Using T3 alone would miss most early cases. TSH is far more sensitive; free T4 assesses severity.

Model Answer: This "dissociated" pattern suggests either: (1) TSH-secreting pituitary adenoma - autonomous TSH overproduction drives T4 hypersecretion; (2) Thyroid hormone resistance (Refetoff's syndrome) - mutated TRβ, pituitary and peripheral tissues resist T4, pituitary keeps secreting TSH to drive T4 higher. Differentiating: measure alpha-subunit of TSH (elevated in TSHoma), pituitary MRI (for adenoma), and check for family history + molecular genetic testing (for RTH).

Model Answer: The patient takes a tracer dose of ¹²³I orally; uptake in the thyroid is measured at 24 hours. Normal: 10-30%. Interpretation:

• High uptake (>50%): Graves' disease, toxic multinodular goiter, toxic adenoma - intrinsic gland overactivity
• Low/suppressed uptake (<5%): Thyroiditis (subacute, postpartum, factitious hyperthyroidism) - exogenous or destructive release of hormone, no active trapping This distinction is critical because treatment differs: antithyroid drugs/RAI for high-uptake; NSAIDs/steroids and symptom management for thyroiditis.

Model Answer: TSI (also called TSHR-stimulating antibodies or TRAb) are IgG antibodies directed against the TSH receptor on thyroid follicular cells. They mimic TSH action, constitutively activating adenylyl cyclase, causing unregulated thyroid hormone production. TSI is the pathogenic antibody in Graves' disease and also explains Graves' ophthalmopathy (TSHR expressed in orbital fibroblasts). TSI also crosses the placenta, causing neonatal Graves' disease in newborns of affected mothers. Testing TSI is useful in pregnancy and in ambiguous cases. (Schwartz's Principles of Surgery, 11e)

Model Answer: Heterophilic antibodies (human anti-animal antibodies) can interfere with immunoassays. In immunometric (sandwich) assays for TSH, they can cross-link the capture and signal antibodies even in the absence of TSH, giving falsely elevated TSH. In competitive immunoassays for T4/T3, they can block antibody-hormone binding, giving falsely elevated T4/T3. Recognition matters because these patients can be mistakenly treated for thyroid disease. Suspicion arises when clinical picture doesn't match biochemistry; serial dilution or blocking reagents confirm interference.

Model Answer: In severe systemic illness (ICU, surgery, starvation, sepsis), peripheral deiodinase activity is altered: type-1 deiodinase (T4→T3 conversion) is suppressed and type-3 deiodinase (T4→rT3) is upregulated. Result:

• Low total T3 and free T3 (commonest finding - "low T3 syndrome")
• Reverse T3 (rT3) elevated
• T4 and TSH may be low in severe illness This is an adaptive response to conserve catabolism. Treating these patients with T4 is not beneficial and may worsen outcomes. TSH and free T4 normalize on recovery.

Model Answer: Both show low TSH and low free T4 in an ill patient. Key distinguishing features:

• Sick euthyroid: no prior pituitary/hypothalamic disease, TSH often still detectable (not zero), rT3 is elevated, free T4 may recover after illness resolves, no features of hypopituitarism
• Secondary hypothyroidism: prior pituitary disease/surgery, TSH inappropriately low (but can be mildly elevated in TRH deficiency), associated hormonal deficiencies (cortisol, GH), rT3 normal Dynamic testing (TRH stimulation) and clinical context differentiate them.

Model Answer: In early or mild hyperthyroidism, free T3 rises before free T4. This is because: (1) Hyperstimulated thyroid preferentially secretes T3 over T4 (2) Elevated TSH (in cases of TSHoma) or TSI (Graves') drives preferential T3 synthesis (3) Peripheral conversion of T4 to T3 is accelerated Free T3 is therefore the first test to become abnormal in subclinical/early hyperthyroidism, though TSH remains the most sensitive initial screen. (Schwartz's Principles of Surgery, 11e)

Model Answer: FTI = Total T4 × T3 Resin Uptake (expressed as a ratio). It corrects for TBG abnormalities mathematically. Still used when: (1) direct free T4 assay is unavailable, (2) albumin abnormalities interfere with equilibrium dialysis assays, (3) familial dysalbuminemia - a condition where abnormal albumin has high affinity for T4, falsely elevating free T4 on many direct assays, but FTI is calculated from total T4 and corrects for this.

Model Answer: Subclinical hyperthyroidism. TSH is suppressed because the pituitary is sensing an excess free thyroid hormone (or is being autonomously overridden), but the peripheral conversion / binding is buffering the free T4 within assay normal limits. Causes: (1) Exogenous: excess thyroid hormone replacement (T4 over-replacement - most common), (2) Endogenous: autonomous thyroid tissue - early Graves', toxic adenoma, toxic multinodular goiter, or physiological in first trimester pregnancy (hCG cross-reacts with TSH receptor).

Model Answer: hCG shares structural homology with TSH (both have identical alpha subunits). At high hCG levels in the first trimester (peak 8-12 weeks), hCG binds and weakly stimulates TSH receptors, driving T4 production. This causes: suppressed TSH, mildly elevated free T4, and sometimes clinical symptoms of hyperthyroidism (hyperemesis gravidarum). It is transient, peaks at 10-12 weeks and resolves by 14-18 weeks as hCG falls. No antithyroid drug treatment needed; supportive management only.

Model Answer: FNAC is the gold standard for evaluating thyroid nodules detected on imaging. Function tests (TSH, T3, T4) tell you if the gland is functioning abnormally, but cannot determine the histological nature of a nodule. FNAC provides cytological diagnosis: benign (colloid cyst, thyroiditis), suspicious/indeterminate (follicular neoplasm), or malignant (papillary carcinoma). Indication: all cold nodules on RAI scan >1 cm. Hot nodules rarely need FNA (malignancy risk <5%). FNAC bridges the gap between biochemistry and pathology.

Model Answer: In Hashimoto's thyroiditis, early disease may show normal or even elevated uptake on RAI scan (due to TSH overstimulation from early hypothyroidism), yet anti-TPO antibodies are markedly elevated and the patient may be clinically hypothyroid. Later in the disease, uptake becomes low and patchy as the gland is destroyed. This mismatch - where functional tests suggest normal/increased activity but antibodies reveal destructive autoimmunity - can mislead clinicians into missing early Hashimoto's.

Model Answer: In primary hypothyroidism, the pituitary-thyroid axis is intact. TSH reliably reflects whether the dose of T4 is sufficient (TSH in normal range = adequate replacement). In secondary (central) hypothyroidism (pituitary/hypothalamic disease), the pituitary cannot produce normal TSH; TSH is already low or unmeasurable regardless of T4 levels. Monitoring on TSH alone would be misleading. Instead, free T4 is the monitoring parameter for secondary hypothyroidism - target mid-to-upper normal range.

Model Answer: The Wolff-Chaikoff effect: a sudden large iodine load transiently inhibits thyroid hormone synthesis (organification block) as a protective mechanism. In most people, the thyroid "escapes" this block within 1-2 weeks. In vulnerable individuals (pre-existing thyroid disease, Hashimoto's, or post-RAI), the block persists, causing iodine-induced hypothyroidism. Relevant to testing: patients who receive iodinated contrast media should have thyroid function tests checked 4-6 weeks later if they have known thyroid disease, as contrast can precipitate hypothyroidism or (in nodular goiter) hyperthyroidism (Jod-Basedow effect).

Model Answer: Jod-Basedow (iodine-induced hyperthyroidism): excess iodine in a patient with autonomous thyroid tissue (toxic MNG, autonomous adenoma, endemic goiter) triggers unregulated T4/T3 synthesis - these autonomous cells cannot downregulate. Pattern: suppressed TSH + elevated free T4 and/or free T3 after iodine exposure (contrast, amiodarone, seaweed). Unlike Wolff-Chaikoff in the normal gland, Jod-Basedow occurs in those with pre-existing autonomy who cannot defend against excess substrate.

Model Answer: Amiodarone (37% iodine by weight) causes multiple effects: (1) Inhibits type-1 deiodinase → reduces T4 → T3 conversion → T3 falls, T4 rises (2) Inhibits T4 entry into cells → T4 accumulates in blood (3) Directly inhibits TSH secretion transiently Early in therapy: TSH may be transiently elevated as T3 falls. Later in chronic use: elevated T4 + low-normal T3 + normal TSH is expected and does NOT indicate hyperthyroidism. Diagnosis of amiodarone-induced thyrotoxicosis requires suppressed TSH + clearly elevated free T3.

Model Answer: Thyroglobulin (Tg) is a 660-kDa glycoprotein produced exclusively by thyroid follicular cells, serving as the scaffold for T3/T4 synthesis. Clinical uses: (1) Tumor marker for differentiated thyroid cancer (papillary, follicular): after total thyroidectomy + RAI ablation, any detectable Tg signals residual/recurrent disease. Rising Tg = disease progression (2) Differentiating exogenous thyrotoxicosis (thyrotoxicosis factitia) from endogenous: if patient is taking excess T4/T3 secretly, TSH is suppressed, T4 elevated, but Tg is low/undetectable (no intrinsic gland activity). In Graves'/thyroiditis, Tg is elevated. (Schwartz's Principles of Surgery, 11e)

Model Answer: After 500 μg IV TRH, measure TSH at 0, 30, 60 minutes:

• Primary hypothyroidism: Exaggerated and prolonged TSH rise (>30 μIU/mL) - pituitary is chronically primed by high endogenous TRH and low T4 feedback
• Secondary hypothyroidism (pituitary failure): Flat or absent TSH response - pituitary thyrotrophs are damaged/absent
• Tertiary hypothyroidism (hypothalamic): Delayed TSH rise (peak at 60-90 min instead of 30 min) - intact but sluggish pituitary
• Hyperthyroidism: Flat response - high T4/T3 suppress pituitary thyrotrophs (Schwartz's Principles of Surgery, 11e)

PART B: ADRENAL GLAND FUNCTION TESTS

40 Viva Questions

Model Answer: Cortisol follows a circadian rhythm driven by the SCN (suprachiasmatic nucleus) via the hypothalamic-pituitary-adrenal (HPA) axis. Highest levels: 6-8 AM (peak ~500-700 nmol/L); lowest levels: midnight (~50-100 nmol/L). Clinical significance: loss of this diurnal variation is the earliest biochemical marker of Cushing's syndrome. A single morning cortisol may be normal in Cushing's, but midnight cortisol will be inappropriately elevated. Tests exploit this by measuring midnight cortisol or using late-night salivary cortisol. (Harper's Biochemistry, 32e)

Model Answer: 1 mg dexamethasone (potent synthetic glucocorticoid) is given orally at 11 PM-midnight. Serum cortisol is measured at 8 AM next morning. Principle: dexamethasone suppresses pituitary ACTH via negative feedback → cortisol should fall. Normal response (suppression): morning cortisol <50 nmol/L (<1.8 μg/dL) - HPA axis is intact. Failure to suppress (cortisol ≥50 nmol/L): suggests autonomous cortisol production (Cushing's syndrome). This is the best screening test for Cushing's. (Harper's Biochemistry, 32e)

Model Answer: False positives (failure to suppress despite no true Cushing's):

1. Pseudo-Cushing's states: alcoholism, severe depression, obesity, poorly controlled diabetes - HPA axis is activated by stress/glucocorticoid resistance
2. Drugs increasing dexamethasone metabolism: phenytoin, rifampicin, carbamazepine (CYP3A4 inducers) - dexamethasone is metabolized before it can suppress
3. Elevated CBG (cortisol-binding globulin): estrogen/OCP use → high total cortisol even if free cortisol is normal
4. Non-compliance or poor absorption of dexamethasone Confirmatory tests include 24-hour UFC (urinary free cortisol) and CRH stimulation.

Model Answer:

• Low-dose DST (LDDST): 0.5 mg dexamethasone every 6 hours for 2 days. Confirms Cushing's syndrome (normal individuals suppress; Cushing's patients of all types do not suppress). It answers: "Does this patient have Cushing's syndrome?"
• High-dose DST (HDDST): 2 mg dexamethasone every 6 hours for 2 days (total 8 mg). Pituitary Cushing's (Cushing's disease, ACTH-dependent) shows >50% suppression; adrenal adenoma and ectopic ACTH tumors do not suppress. It answers: "What is the source? Pituitary vs. adrenal/ectopic?" Combined with ACTH levels and CRH test, these localize the cause.

Model Answer: ACTH measurement is the key step in classifying Cushing's after confirming excess cortisol:

• ACTH undetectable/very low (<5 pg/mL): ACTH-independent Cushing's → adrenal source (adenoma, carcinoma, bilateral hyperplasia)
• ACTH normal or elevated (>15-20 pg/mL): ACTH-dependent Cushing's. Further differentiation needed:
  • Markedly elevated ACTH (>200 pg/mL) with no suppression on HDDST → Ectopic ACTH syndrome (SCLC, carcinoid)
  • Mildly-moderately elevated ACTH with partial suppression on HDDST → Cushing's disease (pituitary corticotroph adenoma)

Model Answer: CRH (100 μg IV) is administered and ACTH + cortisol are measured at -15, 0, +15, +30, +45, +60 minutes. Responses:

• Cushing's disease (pituitary adenoma): Exaggerated ACTH and cortisol rise (≥35% increase in ACTH, ≥20% in cortisol) - the adenoma retains some CRH responsiveness
• Ectopic ACTH tumor: No or minimal response - the tumor secretes ACTH autonomously, independent of CRH
• Adrenal Cushing's: No ACTH response (suppressed ACTH); cortisol may rise minimally Combined with HDDST, the CRH test achieves >90% accuracy in differentiation.

Model Answer: Synthetic ACTH (250 μg IV or IM) is given; cortisol is measured at 0 and 30/60 minutes. Normal response: cortisol rises by ≥200 nmol/L above baseline and/or reaches an absolute peak ≥500 nmol/L (some laboratories: ≥550 nmol/L). Failure to respond indicates adrenocortical insufficiency (either primary - Addison's disease - or secondary after chronic exogenous glucocorticoid use causing adrenal atrophy). The test distinguishes adrenal insufficiency from intact HPA axis; it cannot reliably diagnose early/mild secondary insufficiency. (Tietz Textbook of Laboratory Medicine, 7e)

Model Answer: Low-dose Synacthen uses 1 μg ACTH (vs. standard 250 μg). At physiologic ACTH concentrations, a partially compromised adrenal cortex may still respond to the supraphysiologic 250 μg dose, falsely suggesting sufficiency. The 1 μg test is more sensitive for detecting partial/mild adrenocortical insufficiency - particularly early secondary insufficiency (pituitary disease, exogenous steroids). However, the 1 μg dose is pharmacologically unstable, requires careful preparation, and normal cutoffs vary between centers, limiting widespread standardization. (Tietz Textbook of Laboratory Medicine, 7e)

Model Answer: A morning serum cortisol (measured 8-9 AM) has direct diagnostic utility:

• <100 nmol/L (<3 μg/dL): Very likely adrenal insufficiency - no need for Synacthen test
• >500 nmol/L (>18 μg/dL): Adrenal insufficiency essentially excluded in a stressed/acutely ill patient
• 100-500 nmol/L: Indeterminate - Synacthen stimulation test required In the emergency setting with clinical suspicion of Addisonian crisis: draw cortisol, give hydrocortisone immediately - do not wait for results. ACTH can be measured on the same sample later to guide further workup. (Textbook of Family Medicine, 9e)

Model Answer:

Model Answer: Aldosterone (the main mineralocorticoid regulating K+ excretion) is produced in the zona glomerulosa and is regulated primarily by angiotensin II (RAAS) and serum K+, not by ACTH. In primary Addison's disease, the entire adrenal cortex is destroyed, so aldosterone production is also lost → reduced Na+/K+ exchange in the collecting duct → Na+ loss + K+ retention = hyperkalemia. In secondary adrenal insufficiency (ACTH deficiency alone), the zona glomerulosa is structurally intact and RAAS continues to drive aldosterone secretion → potassium is normally excreted → no hyperkalemia.

Model Answer: Metyrapone blocks 11β-hydroxylase, the final enzyme in cortisol synthesis (converting 11-deoxycortisol to cortisol). Cortisol falls, removing negative feedback → pituitary releases more ACTH → drives adrenal steroidogenesis to produce 11-deoxycortisol (which accumulates) → measured as urinary 17-hydroxycorticoids or serum 11-deoxycortisol. A twofold or greater increase in 11-deoxycortisol = normal pituitary-adrenal reserve. A blunted response indicates pituitary insufficiency (cannot increase ACTH) or adrenal insufficiency. It specifically tests pituitary ACTH reserve. (Katzung's Basic and Clinical Pharmacology, 16e)

Model Answer: Metyrapone blocks cortisol synthesis. In a patient with borderline or partial adrenal insufficiency, this can precipitate an acute Addisonian crisis with vascular collapse. The test is therefore contraindicated in: (1) known or strongly suspected adrenal insufficiency, (2) critically ill patients, (3) late-stage pituitary disease. The test must always be done with emergency hydrocortisone available at bedside. Short-term metyrapone tests (single overnight dose) are safer and have largely replaced the longer protocols. (Katzung's Basic and Clinical Pharmacology, 16e)

Model Answer: A timed 24-hour urine collection is assayed for free (unbound) cortisol by radioimmunoassay or HPLC. Normal: 10-100 μg/24h (varies by lab). Advantages:

• Integrates cortisol production over 24 hours, avoiding single-point sampling variability
• Measures only biologically active free cortisol (unaffected by CBG/albumin changes)
• Not affected by diurnal variation if collected correctly
• Three-fold elevation above normal is highly specific for Cushing's syndrome Disadvantage: incomplete collection invalidates results; adequate collection verified by measuring 24h urinary creatinine (expected: 1-1.5 g/day in adults).

Model Answer: Cortisol in saliva represents the free (unbound) fraction, as CBG is absent from saliva. Salivary cortisol mirrors free serum cortisol. Collected between 11 PM-midnight (the physiological nadir), elevated salivary cortisol (>145 ng/dL by most labs) indicates loss of diurnal rhythm - an early and sensitive marker of Cushing's syndrome. Advantages: non-invasive, can be collected at home by the patient, avoids the stress of phlebotomy (which can elevate serum cortisol), and is highly sensitive and specific (>90%) for Cushing's. Two separate samples on different nights improve accuracy.

Model Answer: IPSS is the gold standard when other tests are equivocal. Bilateral petrosal sinuses (draining the pituitary) and a peripheral vein are cannulated simultaneously. Samples are drawn before and after CRH administration (100 μg IV). A petrosal-to-peripheral ACTH ratio ≥2 basal or ≥3 post-CRH confirms pituitary source (Cushing's disease). A ratio <2 indicates ectopic ACTH source. Additionally, the side with the higher ratio (right vs. left petrosal) helps localize the pituitary microadenoma - important as MRI may miss tumors <5 mm.

Model Answer:

• Cortisol: elevated (morning and midnight)
• ACTH: suppressed/undetectable (autonomous cortisol from adenoma suppresses pituitary)
• No suppression on low-dose or high-dose DST (autonomous secretion)
• Aldosterone: normal (adenoma usually selectively produces cortisol)
• Androgens (DHEA-S): often suppressed (because ACTH is suppressed, the zona reticularis is not stimulated)
• 24h UFC: elevated
• Contralateral adrenal: atrophied (disuse due to chronic ACTH suppression)

Model Answer: After bilateral adrenalectomy for Cushing's disease (ACTH-dependent bilateral adrenal hyperplasia), the removal of cortisol's negative feedback causes the existing pituitary corticotroph adenoma to grow aggressively, secreting very high levels of ACTH and its precursor pro-opiomelanocortin (POMC) - including MSH peptides. Clinical result: skin hyperpigmentation (ACTH/MSH effect), mass effects of a growing pituitary tumor (headache, visual field defects). Biochemical detection: markedly elevated plasma ACTH (often >1000 pg/mL) without corresponding cortisol elevation.

Model Answer: Primary hyperaldosteronism (adrenal overproduction of aldosterone independent of renin):

• Aldosterone elevated (>10-15 ng/dL sitting)
• Renin suppressed (RAAS suppressed by volume expansion from aldosterone)
• ARR >30 (aldosterone [ng/dL] ÷ renin [ng/mL/h]) is the best screening test; ARR >50 with aldosterone >15 ng/dL is highly specific Confirmation tests: saline infusion test (aldosterone fails to suppress below 10 ng/dL), fludrocortisone suppression test. After confirmation, adrenal CT + adrenal vein sampling differentiates bilateral hyperplasia (treat medically with spironolactone) from unilateral adenoma (treat surgically).

Model Answer: PRA measures the rate of angiotensin I generation from endogenous angiotensinogen by renin in plasma. Normal: 0.5-3.5 ng/mL/h (supine). Interpretation:

• High PRA + high aldosterone: Secondary hyperaldosteronism (renal artery stenosis, heart failure, cirrhosis - RAAS appropriately activated)
• Low PRA + high aldosterone: Primary hyperaldosteronism (autonomous aldosterone suppresses renin)
• Low PRA + low aldosterone: Exogenous mineralocorticoid use, Liddle syndrome, 11β-hydroxylase or 17α-hydroxylase excess PRA is the key test to distinguish primary from secondary hyperaldosteronism.

Model Answer:

Model Answer: In CAH, baseline 17-OHP may be borderline (especially in non-classic/late-onset forms). The Synacthen-stimulated 17-OHP test is the gold standard: 250 μg Synacthen IV; measure 17-OHP at 0 and 60 minutes. In classic 21-OHase CAH: basal 17-OHP >10,000 ng/dL. In non-classic CAH: stimulated 17-OHP >10,000 ng/dL (basal may be normal or borderline). Normal: stimulated 17-OHP <1,000 ng/dL. This test discriminates carriers and non-classic cases from normals and guides genetic counseling.

Model Answer: Pheochromocytoma (chromaffin cell tumor of adrenal medulla) secretes catecholamines (epinephrine, norepinephrine):

• Plasma free metanephrines (metanephrine + normetanephrine): Best test - sensitivity ~96%, specificity ~85%. Catecholamines are continuously metabolized to metanephrines even in non-secreting tumors
• 24-hour urinary fractionated metanephrines and catecholamines: High sensitivity and specificity
• 24-hour urinary VMA (vanillylmandelic acid): Less sensitive (50-60%), now less used
• Clonidine suppression test: IV clonidine suppresses sympathetic catecholamines in essential hypertension but NOT in pheochromocytoma. Used when plasma metanephrines are borderline. (Sabiston Textbook of Surgery; Smith & Tanagho's General Urology)

Model Answer: Catecholamines are secreted episodically (in "spells"), so a single blood draw often misses surges. In contrast, metanephrines are produced continuously by intratumoral catechol-O-methyltransferase (COMT) activity - even between secretory episodes. This means metanephrines are elevated throughout the day, providing a stable, reliable marker. Additionally, metanephrines have a longer half-life than catecholamines. False positives occur with sympathomimetics, tricyclics, and physical stress; samples should be collected supine after 30 minutes rest.

Model Answer: Clonidine (α2-agonist) suppresses central sympathetic outflow, reducing catecholamine release from normal chromaffin tissue and nerve terminals. Protocol: measure plasma catecholamines/normetanephrine before and 3 hours after 0.3 mg oral clonidine. Normal response: norepinephrine and normetanephrine fall to normal. In pheochromocytoma: levels remain elevated (autonomous secretion not dependent on sympathetic tone). Used when plasma metanephrines are equivocal. A failure to suppress normetanephrine to <112 pg/mL has >90% specificity for pheochromocytoma.

Model Answer: DHEA-S is the sulfated form of DHEA, produced almost exclusively by the zona reticularis of the adrenal cortex (99%), regulated by ACTH. It is the most abundant circulating steroid hormone. Clinical uses:

• Elevated DHEA-S: excess adrenal androgen production - adrenal Cushing's (carcinoma > adenoma), CAH (21-OHase or 11β-OHase deficiency), adrenal carcinoma, polycystic ovary syndrome (though ovarian DHEA-S is low)
• Low DHEA-S: adrenal insufficiency, aging (adrenopause), hypopituitarism
• Useful to distinguish adrenal vs. ovarian androgen excess in virilizing syndromes (DHEA-S primarily adrenal; testosterone primarily ovarian)

Model Answer: Critically ill patients pose specific challenges:

1. Cortisol is highly protein-bound; acute illness reduces albumin → total cortisol is falsely low even when free cortisol is adequate
2. Stress response elevates ACTH and cortisol; "normal" cortisol in an unstable ICU patient may be inappropriately low
3. Standard Synacthen cutoffs (≥500 nmol/L) may be unreliable due to albumin-lowering effect
4. A delta-cortisol (rise after Synacthen ≥250 nmol/L) or random cortisol <276 nmol/L suggests relative adrenal insufficiency of critical illness (RACI) The CORTICUS trial showed empiric hydrocortisone therapy did not improve mortality unless patients were in septic shock refractory to vasopressors. (Barash Clinical Anesthesia, 9e)

Model Answer: At physiological concentrations, cortisol has minimal mineralocorticoid activity because the enzyme 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2) in renal tubular cells converts cortisol to inactive cortisone, protecting mineralocorticoid receptors. However, in Cushing's syndrome (very high cortisol), the enzyme is overwhelmed; cortisol directly activates aldosterone receptors → hypertension, hypokalemia, metabolic alkalosis. This is also exploited in Apparent Mineralocorticoid Excess (AME) syndrome where 11β-HSD2 is deficient, and in licorice ingestion (glycyrrhizin inhibits 11β-HSD2).

Model Answer: Surgical stress activates the HPA axis via CRH and cytokines, causing a 2-10 fold rise in cortisol. Patients on long-term glucocorticoids or with pituitary/adrenal disease may not mount this response (HPA suppression). Preoperative assessment: Synacthen stimulation test - cortisol must reach ≥500 nmol/L to indicate adequate reserve for surgical stress. If the axis is suppressed: perioperative steroid cover (hydrocortisone 25-100 mg IM/IV at induction + every 8 hours for 24-48h) is given. A morning cortisol >500 nmol/L on the day of surgery generally indicates adequate reserve and no cover is needed. (Barash Clinical Anesthesia, 9e)

Model Answer: An adrenal incidentaloma is an adrenal mass >1 cm found incidentally on imaging. Mandatory biochemical workup regardless of size:

1. Overnight 1 mg DST: rule out subclinical Cushing's syndrome
2. Plasma free metanephrines or 24h urinary metanephrines: rule out pheochromocytoma (must exclude BEFORE any biopsy or surgery)
3. Aldosterone and PRA/renin: rule out primary hyperaldosteronism (if hypertensive/hypokalemic)
4. DHEA-S, androstenedione, 17-OHP (if features of virilization): rule out adrenal carcinoma Pheochromocytoma must always be excluded first because an unsuspected pheo during surgery is life-threatening.

Model Answer:

• High DHEA-S (zona reticularis mass = high adrenal androgens, unlike most benign adenomas)
• Mixed steroid excess: cortisol + androgens + sometimes mineralocorticoids (adrenocortical carcinomas are inefficient steroidogenic factories - produce multiple intermediates)
• Very high 24h UFC and 17-KS (17-ketosteroids)
• Large mass (>4-6 cm; risk of malignancy rises steeply above 4 cm)
• Elevated urinary steroid metabolome (mass spectrometry): characteristic benign vs. malignant steroid profiles now validated as the most accurate biochemical test for adrenal malignancy
• Serum LDH, AFP may be elevated in large carcinomas

Model Answer: The overnight 1 mg DST is the initial test. Post-dexamethasone cortisol levels define the spectrum:

• <50 nmol/L: No autonomous secretion
• 50-138 nmol/L: Possible autonomous cortisol secretion (ACS) - gray zone
• >138 nmol/L: Autonomous cortisol secretion confirmed Additional supporting tests: suppressed ACTH (<10 pg/mL), elevated 24h UFC, loss of diurnal variation (elevated late-night salivary cortisol). Despite no florid Cushing's signs, these patients have increased cardiovascular risk, osteoporosis, and metabolic syndrome, and may benefit from adrenalectomy.

Model Answer: 2 L of normal saline is infused IV over 4 hours (patient recumbent). Aldosterone is measured before and after. Principle: volume loading normally suppresses aldosterone via reduced renin and volume-mediated feedback.

• Normal: Post-infusion aldosterone <5 ng/dL (fully suppressed)
• Primary hyperaldosteronism: Post-infusion aldosterone remains >10 ng/dL (failure to suppress)
• Gray zone: 5-10 ng/dL (requires clinical correlation) Contraindicated in: severe hypertension, heart failure, severe hypokalemia. Fludrocortisone suppression test is an alternative confirmatory test.

Model Answer: AVS involves bilateral cannulation of adrenal veins with simultaneous cortisol (for catheter placement verification) and aldosterone sampling before and after cosyntropin stimulation. Used to lateralize aldosterone excess in primary hyperaldosteronism. Critical because:

• CT has ~40-50% false lateralization rate (may misidentify the wrong side, or miss bilateral disease)
• A unilateral adenoma on CT could coexist with contralateral hyperplasia contributing excess aldosterone
• Operating on the wrong adrenal is catastrophic; AVS prevents this A selectivity index (adrenal cortisol/peripheral cortisol >3x) confirms successful cannulation; lateralization index >4:1 indicates unilateral source.

Model Answer: Insulin (0.15 U/kg IV) induces controlled hypoglycemia (blood glucose <40 mg/dL / 2.2 mmol/L). This physiologic stress maximally activates the hypothalamus → CRH → pituitary ACTH → adrenal cortisol. Normal response: cortisol rises to >500 nmol/L (peak). Failure to respond confirms combined HPA axis insufficiency (at any level). It is the gold standard because it tests the entire HPA axis response to a genuine physiologic stress, not just pharmacologic ACTH stimulation of the adrenal alone (which could miss secondary insufficiency with intact adrenals). Contraindications: epilepsy, ischemic heart disease, very low baseline cortisol.

Model Answer: Standard serum cortisol testing is unhelpful in patients on hydrocortisone (assay cross-reacts and gives falsely elevated results). Monitoring relies on:

1. Clinical assessment: energy, weight, postural BP, electrolytes
2. Serum electrolytes: Na+, K+ (detect under/over-replacement of aldosterone)
3. Renin levels (for mineralocorticoid adequacy): Target high-normal renin in fludrocortisone-replaced patients
4. Morning cortisol before morning hydrocortisone dose: Checks if patient has taken medication; low pre-dose cortisol is expected
5. 24h urinary free cortisol is unreliable with exogenous hydrocortisone
6. ACTH remains elevated in primary Addison's regardless of good replacement (not a monitoring tool)

Model Answer: Glucagon (1 mg IM or SC) stimulates catecholamine release from the adrenal medulla via postganglionic sympathetic nerves, and also stimulates GH and cortisol through indirect mechanisms (hypoglycemia counterregulation). Used when ITT is contraindicated (epilepsy, cardiac disease, high risk of severe hypoglycemia). Protocol: measure cortisol and GH at 0, 90, 120, 150, 180 minutes. Normal cortisol response: ≥500 nmol/L. The glucagon test is also used for pheochromocytoma diagnosis (provocative test - causes catecholamine release in pheo; can cause hypertensive crisis - done only when metanephrines are borderline and with phentolamine available).

Model Answer: The most fundamental defect in Cushing's syndrome (regardless of cause) is loss of the normal circadian cortisol rhythm - specifically, failure of the nighttime nadir. Even in mild Cushing's where morning cortisol may overlap with stressed normals, the midnight cortisol is invariably elevated above normal (<50 nmol/L serum; <145 ng/dL saliva). This is because: (1) autonomous ACTH/cortisol secretion is continuous, not rhythmic; (2) the suprachiasmatic nucleus cannot override the autonomous tumor drive. Thus midnight cortisol has ~90-95% sensitivity for Cushing's and requires two positive results on separate nights for diagnosis.

Model Answer:

Model Answer: Pattern: mineralocorticoid excess with low renin (RAAS suppressed) + normal glucocorticoids.

Quick-Reference Summary Tables

Thyroid Function Test Interpretation Matrix

Adrenal Function Test - Cushing's Localization Protocol

Elevated 24h UFC or Late-night salivary cortisol
            ↓
    1mg overnight DST
    (fails to suppress)
            ↓
    Plasma ACTH
    ↙               ↘
 Low (<5)         High/Normal
 Adrenal          ACTH-dependent
 source                ↓
 CT adrenal      HDDST + CRH test
                  ↙           ↘
          Suppresses        No suppression
          +CRH rises        +CRH flat
          Cushing's         Ectopic ACTH
          Disease           (SCLC, carcinoid)
          ↓
          IPSS for
          lateralization