Generate thyroid and adrenal gland function tests Tutorial mbbs first year batch viva questions university level medium to hard interlinked 40 questions answers first year level
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Thyroid & Adrenal Gland Function Tests
MBBS First Year - Tutorial Viva Questions (University Level, Medium-Hard)
40 Interlinked Q&A
SECTION A: THYROID GLAND - HORMONES & PHYSIOLOGY (Q1-10)
Q1. What hormones does the thyroid gland secrete, and what are their chemical natures?
A. The thyroid gland secretes:
- T4 (thyroxine / tetraiodothyronine) - contains 4 iodine atoms; the major secretory product (~93%)
- T3 (triiodothyronine) - contains 3 iodine atoms; the biologically active form (~7% secreted, rest converted peripherally from T4)
- Calcitonin - secreted by parafollicular C-cells; lowers serum calcium by inhibiting osteoclastic bone resorption
Both T3 and T4 are iodinated derivatives of the amino acid tyrosine.
(Harper's Illustrated Biochemistry, 32nd Ed)
Q2. Describe the hypothalamic-pituitary-thyroid (HPT) axis and its feedback regulation.
A. The HPT axis operates as a classic negative feedback loop:
- Hypothalamus releases TRH (Thyrotropin-Releasing Hormone)
- TRH stimulates the anterior pituitary to secrete TSH (Thyroid-Stimulating Hormone / Thyrotropin)
- TSH stimulates the thyroid gland to produce and release T3 and T4
- Elevated T3/T4 exerts negative feedback on both the hypothalamus and anterior pituitary, suppressing TRH and TSH secretion respectively
This tight loop means small changes in free T3/T4 produce large, disproportionate changes in TSH - which is why TSH is the most sensitive indicator of thyroid status.
(Quick Compendium of Clinical Pathology, 5th Ed; Robbins & Cotran Pathologic Basis of Disease)
Q3. Which is the most sensitive and best initial test for assessing thyroid function, and why?
A. Serum TSH is the best initial test. Reasons:
- Due to the logarithmic nature of the feedback loop, small changes in free T3/T4 produce large changes in TSH
- A normal TSH essentially excludes thyroid dysfunction in most patients
- The standard approach: TSH first → if abnormal, then measure free T4 → measure free T3 in select circumstances (e.g., suspected T3 toxicosis)
- TSH is measured by sensitive third-generation immunoassays
(Quick Compendium of Clinical Pathology, 5th Ed)
Q4. Why is free T4 (fT4) preferred over total T4 in current clinical practice?
A. Total T4 reflects both bound and free fractions. About 99.97% of T4 is protein-bound (to Thyroxine-Binding Globulin [TBG], prealbumin, albumin). Changes in TBG concentration (e.g., pregnancy increases TBG; androgens, nephrotic syndrome decrease TBG) will alter total T4 without any actual thyroid dysfunction. Free T4 measures only the biologically active unbound fraction, which truly reflects thyroid status. Therefore, total T4 is rarely measured nowadays.
(Harper's Illustrated Biochemistry, 32nd Ed)
Q5. What is T3 toxicosis? When is free T3 measurement specifically indicated?
A. T3 toxicosis is hyperthyroidism caused by autonomous overproduction of T3 with normal T4 levels. TSH is suppressed, free T4 is normal, but free T3 is elevated. Free T3 measurement is indicated:
- To diagnose T3 toxicosis
- To detect impaired peripheral conversion of T4 → T3
- To monitor patients on oral T3 therapy (liothyronine)
- To investigate suspected analytical interference with the free T4 assay (Quick Compendium of Clinical Pathology, 5th Ed)
Q6. What is Radioactive Iodine Uptake (RAIU) test? How does it differentiate causes of hyperthyroidism?
A. RAIU is a nuclear medicine test where the patient ingests a dose of radioactive iodine (¹²³I or ¹³¹I); the thyroid is then scanned for radioactivity after 4-24 hours.
| Condition | RAIU |
|---|---|
| Graves' disease | Increased (diffuse) |
| Toxic adenoma | Increased (focal "hot" nodule) |
| Thyroiditis (de Quervain's) | Decreased (gland releasing stored hormone) |
| Struma ovarii | Decreased in neck |
| Exogenous thyroid hormone (factitious) | Decreased |
(Quick Compendium of Clinical Pathology, 5th Ed)
Q7. What are the typical laboratory findings in primary hypothyroidism vs. secondary (central) hypothyroidism?
A.
| Parameter | Primary Hypothyroidism | Secondary (Central) Hypothyroidism |
|---|---|---|
| TSH | HIGH | Low or inappropriately normal |
| Free T4 | Low | Low |
| Free T3 | Low | Low |
| Cause | Thyroid gland failure | Pituitary/hypothalamic failure |
Key point: In secondary/central hypothyroidism, TSH alone is misleading. Diagnosis requires combined assessment of both TSH and free T4. A normal or low TSH with low free T4 points to pituitary/hypothalamic disease.
(Quick Compendium of Clinical Pathology, 5th Ed)
Q8. What is Euthyroid Sick Syndrome (Non-Thyroidal Illness / NTI)? What pattern of thyroid tests is seen?
A. In critically ill patients, thyroid function tests become abnormal without true thyroid disease - this is NTI or Euthyroid Sick Syndrome.
Typical pattern:
- Decreased T4 and T3
- Increased reverse T3 (rT3) - inactive metabolite from T4
- Normal to low TSH
Mechanism: Illness reduces peripheral T4-to-T3 conversion; rT3 (from T4) increases. This mimics central hypothyroidism.
Clinical implication: Thyroid function testing should NOT be routinely done in critically ill patients unless there is strong clinical suspicion of pre-existing thyroid disease.
(Quick Compendium of Clinical Pathology, 5th Ed)
Q9. Why may TSH remain abnormally low even after treatment of hyperthyroidism?
A. Chronic suppression of TSH leads to thyrotroph cell suppression in the anterior pituitary. Even after thyroid hormone levels normalize following treatment (radioiodine, surgery, antithyroid drugs), the thyrotroph cells remain suppressed and take time to recover. TSH may remain low for weeks to months. Similarly, thyrotroph cell hyperplasia in longstanding hypothyroidism may delay TSH normalization after starting T4 replacement. Steady state is typically achieved by end of the second month of treatment in compliant patients.
(Quick Compendium of Clinical Pathology, 5th Ed)
Q10. A patient has elevated TSH with high free T4. What does this suggest?
A. Elevated TSH with high free T4 is a paradoxical finding. It suggests:
- TSH-secreting pituitary adenoma (TSHoma) - a rare cause of hyperthyroidism where the pituitary autonomously secretes TSH despite high T4 levels
- Thyroid hormone resistance syndrome - mutation in the thyroid hormone receptor (usually TRβ) causes resistance to T4/T3 feedback at the pituitary
This contrasts with primary hyperthyroidism where: low TSH + high free T4 is the expected pattern.
(Tintinalli's Emergency Medicine; Goldman-Cecil Medicine)
SECTION B: THYROID PATHOLOGY & CLINICAL CORRELATION (Q11-16)
Q11. What lab pattern confirms primary hyperthyroidism? Name the most common cause.
A.
- Pattern: Low/suppressed TSH + elevated free T4 ± elevated free T3
- Most common cause: Graves' disease (diffuse toxic goitre) - an autoimmune condition caused by TSH-receptor stimulating antibodies (TRAb/TSI) that chronically stimulate the thyroid gland
Secondary findings in Graves' disease: pretibial myxedema (in ~5%), exophthalmos, thyroid bruit.
(Tintinalli's Emergency Medicine)
Q12. What is the significance of measuring anti-TPO antibodies and anti-Tg antibodies in thyroid disease?
A.
- Anti-TPO (anti-thyroid peroxidase): Most sensitive marker for autoimmune thyroid disease; elevated in Hashimoto's thyroiditis and Graves' disease
- Anti-Tg (anti-thyroglobulin): Elevated in autoimmune thyroid disease; also used for monitoring after thyroid cancer surgery (rising anti-Tg may indicate recurrence)
- TRAb (TSH receptor antibodies): Diagnostic and prognostic in Graves' disease; positive indicates autoimmune stimulation of the thyroid
These tests help distinguish autoimmune thyroid disease from other causes of hypo/hyperthyroidism.
Q13. What is the role of serum thyroglobulin in thyroid cancer management?
A. Thyroglobulin (Tg) is produced exclusively by thyroid follicular cells. After total thyroidectomy for differentiated thyroid cancer:
- Serum Tg should be undetectable
- A rising Tg indicates recurrence or residual thyroid tissue
- Tg is measured alongside anti-Tg antibodies (which can interfere with the assay)
- TSH stimulation (by withdrawal of T4 or rhTSH injection) enhances sensitivity of Tg detection
Tg is NOT useful for initial diagnosis of thyroid cancer but is a gold-standard tumor marker for follow-up.
Q14. What is calcitonin's clinical significance? In which thyroid tumor is it markedly elevated?
A. Calcitonin is produced by parafollicular C-cells of the thyroid. It lowers serum calcium by:
- Inhibiting osteoclast activity (blocks bone resorption)
- Promoting renal calcium excretion
Markedly elevated in medullary thyroid carcinoma (MTC) - a tumor of C-cells. Calcitonin is the specific tumor marker for MTC and is used for:
- Screening family members (MTC is associated with MEN2A/MEN2B)
- Monitoring treatment response
- Detecting recurrence
Very high calcitonin levels can cause hypocalcemia.
(Henry's Clinical Diagnosis and Management by Laboratory Methods)
Q15. Why is TBG important, and what conditions alter its concentration?
A. TBG (Thyroxine-Binding Globulin) is the main carrier protein for thyroid hormones in the blood. It binds ~75% of T4 and T3. Changes in TBG alter total (but not free) T4/T3 levels.
TBG increased by: Pregnancy, estrogens/OCPs, hepatitis, genetic TBG excess - causes high total T4 but euthyroid
TBG decreased by: Androgens, glucocorticoids, nephrotic syndrome (protein loss), hepatic failure, starvation - causes low total T4 but euthyroid
This is why free T4 is the preferred test - it is unaffected by TBG changes.
Q16. Classify hypothyroidism by level of the HPT axis and match with TSH/T4 patterns.
A.
| Type | Site of Defect | TSH | Free T4 |
|---|---|---|---|
| Primary | Thyroid gland | HIGH | Low |
| Secondary | Anterior pituitary | Low/normal | Low |
| Tertiary | Hypothalamus (↓TRH) | Low/normal | Low |
Most common cause of primary hypothyroidism worldwide: iodine deficiency. In developed countries: Hashimoto's thyroiditis (autoimmune).
SECTION C: ADRENAL GLAND - HORMONES & PHYSIOLOGY (Q17-22)
Q17. What are the zones of the adrenal cortex and what does each secrete?
A. Using the mnemonic "GFR" (Glomerulosa-Fasciculata-Reticularis):
| Zone | Hormone | Regulator |
|---|---|---|
| Zona Glomerulosa (outer) | Aldosterone (mineralocorticoid) | Renin-Angiotensin system, K⁺ |
| Zona Fasciculata (middle) | Cortisol (glucocorticoid) | ACTH |
| Zona Reticularis (inner) | Adrenal androgens (DHEA, DHEAS) | ACTH |
The adrenal medulla (separate structure) secretes catecholamines: adrenaline (epinephrine, ~80%) and noradrenaline (norepinephrine, ~20%).
Q18. Describe the diurnal variation of cortisol. Why is this physiologically important?
A. Cortisol secretion follows a circadian rhythm driven by the HPA axis:
- Peak: Early morning (6-8 AM) - highest levels (~25 µg/dL)
- Nadir: Around midnight - lowest levels
Physiological importance:
- Prepares the body for the metabolic demands of waking (gluconeogenesis, anti-inflammatory readiness)
- Loss of diurnal variation is one of the earliest signs of adrenal hyperfunction (Cushing syndrome)
Reference ranges (serum cortisol): Morning = ~10-25 µg/dL; Midnight = much lower.
(Henry's Clinical Diagnosis and Management by Laboratory Methods)
Q19. What is the HPA axis? How does it regulate cortisol secretion?
A. The Hypothalamic-Pituitary-Adrenal (HPA) axis:
- Hypothalamus releases CRH (Corticotropin-Releasing Hormone)
- CRH stimulates the anterior pituitary to secrete ACTH (Adrenocorticotropic Hormone)
- ACTH stimulates the zona fasciculata of the adrenal cortex to produce and secrete cortisol
- Elevated cortisol provides negative feedback to both hypothalamus (↓CRH) and pituitary (↓ACTH)
In primary adrenal insufficiency (Addison's): low cortisol → ACTH rises (no inhibition)
In secondary adrenal insufficiency: low ACTH → low cortisol
(Henry's Clinical Diagnosis and Management by Laboratory Methods)
Q20. What is the reference range for serum cortisol, and what is measured in urinary free cortisol? Why is it used?
A.
- Morning serum cortisol reference range: ~10-25 µg/dL (in healthy males)
- Urinary free cortisol (UFC) reference range: ~24-108 µg/24 hours
Rationale for UFC:
- Almost all cortisol is protein-bound in serum (mainly to transcortin/CBG)
- In hypercortisolism, cortisol exceeds transcortin-binding capacity → free (unbound) cortisol spills into urine
- UFC represents integrated 24-hour cortisol production - more reliable than single serum measurements
- Elevated UFC (>3× normal) is highly suggestive of Cushing syndrome
(Henry's Clinical Diagnosis and Management by Laboratory Methods)
Q21. How does aldosterone work and what tests assess the renin-angiotensin-aldosterone system (RAAS)?
A. Aldosterone acts on the collecting duct of the kidney to:
- Retain Na⁺ (and water) → increases blood volume/pressure
- Excrete K⁺ and H⁺
Tests for RAAS:
- Plasma Aldosterone Concentration (PAC) - elevated in hyperaldosteronism
- Plasma Renin Activity (PRA) - elevated in secondary hyperaldosteronism; suppressed in primary (Conn's syndrome)
- Aldosterone-to-Renin Ratio (ARR) - best screening test for primary hyperaldosteronism; elevated ratio (high aldosterone, low renin) indicates autonomous aldosterone secretion
(Tietz Textbook of Laboratory Medicine, 7th Ed)
Q22. What are the actions of cortisol in the body? Link each action to a clinical manifestation when cortisol is in excess.
A.
| Cortisol Action | Excess (Cushing syndrome) Manifestation |
|---|---|
| ↑ Gluconeogenesis | Hyperglycemia / diabetes |
| ↑ Protein catabolism | Muscle wasting, proximal myopathy |
| ↑ Fat redistribution | Central obesity, buffalo hump, moon face |
| Anti-inflammatory / immunosuppression | Increased infection susceptibility |
| ↑ Bone resorption | Osteoporosis, pathological fractures |
| ↑ Sodium retention | Hypertension |
| Anti-insulin effect | Insulin resistance |
| Inhibits gonadotropins | Menstrual irregularities, decreased libido |
SECTION D: ADRENAL FUNCTION TESTS (Q23-32)
Q23. What is the Dexamethasone Suppression Test (DST)? What is its physiological basis?
A. Dexamethasone is a potent synthetic glucocorticoid that strongly suppresses pituitary ACTH secretion via negative feedback.
Physiological basis: In a normal person, giving dexamethasone mimics high cortisol → hypothalamus and pituitary sense high glucocorticoid → suppress ACTH → adrenal cortisol production falls.
In Cushing syndrome (autonomous cortisol production or ACTH-secreting tumor), this suppression is lost or incomplete.
(Harper's Illustrated Biochemistry, 32nd Ed; Henry's Clinical Diagnosis)
Q24. Describe the Low-Dose Dexamethasone Suppression Test (LDDST). What does it diagnose?
A.
- Protocol (overnight): 1 mg dexamethasone administered at midnight → serum cortisol measured at 8 AM next morning
- Normal response: Cortisol suppressed to <1.8 µg/dL (some labs use <5 µg/dL)
- Abnormal (failure to suppress): Cortisol remains elevated → suggests Cushing syndrome
LDDST is a SCREENING test for hypercortisolism. A normal result effectively excludes Cushing syndrome.
Other screening tests: midnight salivary cortisol (×2), 24-hour UFC (×2).
(Current Surgical Therapy, 14th Ed; Harper's Illustrated Biochemistry)
Q25. Describe the High-Dose Dexamethasone Suppression Test (HDDST). What does it localize?
A.
- Protocol: 8 mg dexamethasone given at midnight → cortisol measured at 8 AM
- Purpose: To differentiate causes of hypercortisolism AFTER LDDST confirms Cushing syndrome
| Result | Interpretation |
|---|---|
| Cortisol suppressed (>50% fall) | Cushing disease (pituitary adenoma) - retains some negative feedback |
| No suppression of cortisol | Adrenal adenoma/carcinoma OR ectopic ACTH syndrome |
Note: Ectopic ACTH tumors (small cell lung cancer, etc.) also fail to suppress with high-dose dexamethasone.
(Swanson's Family Medicine Review; Barash's Clinical Anesthesia, 9th Ed)
Q26. Link DST results to ACTH levels to localize the source of hypercortisolism.
A.
| Condition | Cortisol | ACTH | HDDST |
|---|---|---|---|
| Cushing disease (pituitary adenoma) | High | High | Suppressed |
| Adrenal adenoma/carcinoma | High | Low (suppressed) | No suppression |
| Ectopic ACTH (e.g., lung cancer) | High | Very high | No suppression |
Key principle: Measuring plasma ACTH alongside cortisol is essential. Low ACTH + high cortisol = adrenal source. High ACTH + high cortisol = pituitary or ectopic source.
(Henry's Clinical Diagnosis; Costanzo Physiology, 7th Ed)
Q27. What is the ACTH (Cosyntropin/Synacthen) Stimulation Test? Describe the protocol and interpretation.
A. Tests the ability of the adrenal cortex to respond to ACTH stimulation.
Protocol:
- Measure baseline serum cortisol
- Inject 250 µg synthetic ACTH (cosyntropin/tetracosactide) IV/IM
- Measure cortisol at 30 and 60 minutes post-injection
Interpretation:
- Normal response: Cortisol rises to >18-20 µg/dL (>500 nmol/L) at 30-60 min
- Abnormal (insufficient rise): Adrenal insufficiency
Limitations: Cannot reliably distinguish primary from secondary adrenal insufficiency acutely. A prolonged ACTH stimulation test (3-day protocol) helps differentiate: in secondary AI, prolonged ACTH eventually stimulates the atrophied adrenal; in primary AI (Addison's), the adrenal cannot respond even with prolonged stimulation.
(Tietz Textbook of Laboratory Medicine; Textbook of Family Medicine, 9th Ed)
Q28. What are the biochemical features of Addison's Disease (Primary Adrenal Insufficiency)?
A. In Addison's disease, both glucocorticoid (cortisol) and mineralocorticoid (aldosterone) deficiency occur:
Biochemistry:
- Low serum cortisol (morning)
- Elevated ACTH (due to loss of negative feedback)
- Hyponatremia (↓aldosterone → ↓Na⁺ retention)
- Hyperkalemia (↓aldosterone → ↑K⁺ retention)
- Metabolic acidosis (↓H⁺ excretion)
- Hypoglycemia (↓gluconeogenesis)
- ACTH stimulation test: abnormal (adrenal cortex cannot respond)
Most common cause: Autoimmune adrenalitis (~70-90% in developed world). Other causes: TB, hemorrhage, infiltration (malignancy), Waterhouse-Friderichsen syndrome.
(Tietz Textbook of Laboratory Medicine, 7th Ed; Katzung's Pharmacology, 16th Ed)
Q29. What is Waterhouse-Friderichsen Syndrome? How does it cause adrenal insufficiency?
A. Waterhouse-Friderichsen Syndrome is acute bilateral adrenal hemorrhage and infarction, most classically caused by Neisseria meningitidis septicemia (also Pseudomonas, other gram-negative bacteria).
Mechanism: Endotoxin-induced disseminated intravascular coagulation (DIC) → hemorrhage into the adrenal glands → acute adrenal infarction → acute adrenal crisis (life-threatening).
Presentation: Septicemia + rapidly progressing shock + purpuric skin rash + bilateral adrenal hemorrhage on imaging.
(Tietz Textbook of Laboratory Medicine, 7th Ed)
Q30. What drugs can cause adrenal insufficiency by inhibiting adrenal steroidogenesis?
A. Several drugs inhibit key steroidogenic enzymes in the adrenal cortex:
- Antifungals: Ketoconazole, fluconazole, posaconazole (inhibit CYP enzymes in steroid synthesis)
- Sedative: Etomidate (inhibits 11β-hydroxylase; even a single dose can suppress cortisol in ICU patients)
- Antibiotic: Rifampicin (induces cortisol metabolism + inhibits synthesis)
- Anticancer agents: Mitotane, abiraterone, enzalutamide
This is clinically important when interpreting cortisol levels in patients on these medications.
(Tietz Textbook of Laboratory Medicine, 7th Ed)
Q31. Compare primary vs. secondary vs. tertiary adrenal insufficiency - biochemical patterns.
A.
| Parameter | Primary (Addison's) | Secondary (Pituitary failure) | Tertiary (Hypothalamic) |
|---|---|---|---|
| Cortisol | Low | Low | Low |
| ACTH | HIGH | Low | Low |
| Aldosterone | Low | Normal | Normal |
| Renin | High | Normal | Normal |
| Cause | Adrenal gland destroyed | ↓ACTH from pituitary | ↓CRH from hypothalamus |
| Hyperpigmentation | Yes (↑ACTH/MSH) | No | No |
In secondary and tertiary AI, mineralocorticoid function is preserved (aldosterone is regulated by RAAS, not ACTH).
(Henry's Clinical Diagnosis and Management by Laboratory Methods)
Q32. What is the role of midnight salivary cortisol in diagnosing Cushing syndrome?
A. Since cortisol is at its nadir around midnight in normal individuals, a midnight salivary cortisol that is elevated indicates loss of the normal diurnal rhythm - the hallmark of Cushing syndrome.
Advantages:
- Non-invasive (no blood draw needed)
- Salivary cortisol correlates with free plasma cortisol
- Patient can collect the sample at home
- Two consecutive elevated midnight salivary cortisol values are needed (to account for stress-related single elevations)
Sensitivity ~93-100%, specificity ~93-100% in most studies.
(Current Surgical Therapy, 14th Ed; Harper's Illustrated Biochemistry)
SECTION E: INTEGRATED & INTERLINKED CLINICAL VIVA QUESTIONS (Q33-40)
Q33. A patient has hypertension, hypokalemia, and low plasma renin with high aldosterone. What is the diagnosis, and what tests confirm it?
A. This is Primary Hyperaldosteronism (Conn's Syndrome).
Pathophysiology: Autonomous aldosterone secretion (usually from adrenal adenoma) → independent of RAAS → Na⁺ and water retention (hypertension) + K⁺ excretion (hypokalemia) + suppressed renin (negative feedback from high BP and volume expansion).
Confirmatory tests:
- Aldosterone-to-Renin Ratio (ARR) - elevated (>30 with PAC >15 ng/dL)
- Salt loading test (IV or oral) - failure to suppress aldosterone confirms autonomous secretion
- Adrenal CT/MRI - to localize adenoma
- Adrenal vein sampling - gold standard to confirm unilateral vs. bilateral disease
(Tietz Textbook of Laboratory Medicine; Campbell Walsh Wein Urology)
Q34. Connect the following: A patient on long-term steroids for asthma undergoes surgery. Why might they develop hypotension intraoperatively, and what tests would you order?
A. This is iatrogenic secondary adrenal insufficiency (tertiary AI).
Mechanism: Prolonged exogenous glucocorticoid administration suppresses the HPA axis via negative feedback → hypothalamus and pituitary stop producing CRH and ACTH → adrenal cortex atrophies. When steroids are abruptly stopped or the patient faces surgical stress, the adrenal cannot mount an adequate cortisol stress response → adrenal crisis with hypotension.
Tests:
- Morning serum cortisol (low)
- ACTH level (low - unlike Addison's where ACTH is high)
- ACTH stimulation test (poor cortisol response initially; may improve with prolonged ACTH due to adrenal atrophy - not gland destruction)
Management: Perioperative "stress dose" steroid coverage.
(Henry's Clinical Diagnosis; Tietz Textbook)
Q35. A young woman presents with weight gain, moon face, and striae. TSH is normal. What hormonal investigations would you do next, and in what sequence?
A. Normal TSH rules out hypothyroidism as the cause. The presentation suggests Cushing syndrome.
Stepwise investigation:
-
Screening tests (any one):
- 24-hour urinary free cortisol (×2 collections)
- Overnight 1 mg LDDST (measure cortisol at 8 AM; >1.8 µg/dL = abnormal)
- Midnight salivary cortisol (×2)
-
If screening positive - confirm source:
- Plasma ACTH level
- ACTH low: Adrenal source → adrenal CT
- ACTH high: Pituitary or ectopic source → High-dose DST (8 mg)
- Suppressed → Cushing disease (pituitary) → pituitary MRI
- Not suppressed → Ectopic ACTH → chest CT/octreotide scan
- Plasma ACTH level
(Swanson's Family Medicine Review; Current Surgical Therapy)
Q36. How does TSH measurement help in monitoring thyroid cancer after thyroidectomy?
A. After total thyroidectomy for differentiated thyroid cancer (papillary/follicular):
- Patients receive TSH-suppressive T4 therapy (high-dose levothyroxine) to keep TSH very low (< 0.1 mU/L) - because TSH promotes growth of any residual differentiated thyroid cancer cells
- Serum thyroglobulin (Tg) is monitored as tumor marker
- For Tg testing, TSH is elevated (by withdrawing T4 for 4-6 weeks or using recombinant human TSH injection) to maximally stimulate Tg production from any residual tissue
- Rising Tg during TSH suppression indicates recurrence
This is an example of how TSH manipulation (both suppression and stimulation) is therapeutically important.
Q37. What is the biochemical connection between the thyroid and adrenal axes in critical illness?
A. In critical illness (sepsis, major surgery, trauma), BOTH axes are affected simultaneously:
Thyroid (NTI/Euthyroid Sick Syndrome):
- ↓ T3 (impaired T4→T3 conversion by peripheral deiodinase inhibition)
- ↑ Reverse T3 (rT3)
- Normal to low TSH
Adrenal (Critical Illness-Related Corticosteroid Insufficiency / CIRCI):
- Relative adrenal insufficiency - ACTH stimulation test may show poor response
- Cortisol levels may appear normal but are insufficient for the degree of stress
Interlink: Both changes may represent adaptive/protective responses to severe illness. Routine replacement of thyroid hormone in NTI is NOT recommended. Steroid replacement in CIRCI is controversial (only in refractory septic shock per current guidelines). These patterns must NOT be mistaken for primary thyroid or adrenal disease.
Q38. A patient has hyponatremia. How do you use hormonal tests to differentiate SIADH from adrenal insufficiency as the cause?
A. Both SIADH and adrenal insufficiency can cause hyponatremia, but the mechanism and management differ completely.
Distinguishing features:
| Parameter | SIADH | Adrenal Insufficiency |
|---|---|---|
| Serum cortisol | Normal | Low |
| Plasma ACTH | Normal | High (primary AI) / Low (secondary AI) |
| Blood pressure | Usually normal | Hypotension |
| Potassium | Normal | Hyperkalemia (primary AI) |
| ACTH stimulation test | Normal | Abnormal |
| BUN/Creatinine | Low (dilution) | Elevated (dehydration) |
Key: Always measure morning cortisol and ACTH in hyponatremia before labeling it SIADH - missing adrenal insufficiency is dangerous.
Q39. How are thyroid function tests affected by pregnancy? Link this to TBG physiology.
A. Pregnancy causes multiple changes in thyroid tests:
- Estrogen → increases hepatic TBG synthesis → higher TBG levels
- Higher TBG binds more T4 and T3 → total T4 and total T3 rise (by ~50%)
- Free T4 and T3 remain normal (or slightly decrease in late pregnancy)
- TSH: Slightly decreases in first trimester (due to hCG cross-reacting with TSH receptors → mild thyroid stimulation → transient gestational hyperthyroidism)
- Iodine requirement increases (fetal demand + increased renal clearance)
Clinical implication: Total T4 levels are unreliable in pregnancy. Free T4 + TSH must be used. TSH reference ranges are trimester-specific in pregnancy.
(Harper's Illustrated Biochemistry, 32nd Ed)
Q40. Summarize the first-line investigation to order for each of the following clinical scenarios, linking thyroid and adrenal tests:
(a) Unexplained weight gain, fatigue, constipation | (b) Unexplained weight loss, palpitations, heat intolerance | (c) Moon face, central obesity, hypertension, striae | (d) Hypotension, hyperpigmentation, hyponatremia, hyperkalemia | (e) Hypertension, hypokalemia, suppressed renin
A.
| Scenario | Likely Diagnosis | First-Line Test(s) |
|---|---|---|
| (a) Weight gain, fatigue, constipation | Primary hypothyroidism | Serum TSH (then free T4 if abnormal) |
| (b) Weight loss, palpitations, heat intolerance | Hyperthyroidism (Graves') | Serum TSH (suppressed) + free T4 |
| (c) Moon face, striae, central obesity | Cushing syndrome | Overnight 1 mg DST + 24-hr UFC |
| (d) Hypotension, hyperpigmentation, hyponatremia/hyperkalemia | Addison's Disease (Primary AI) | Morning serum cortisol + ACTH + ACTH stimulation test |
| (e) Hypertension, hypokalemia, ↓ renin | Primary Hyperaldosteronism (Conn's) | Aldosterone-to-Renin Ratio (ARR) |
Quick Reference Summary Table
| Test | What It Measures | Key Use |
|---|---|---|
| TSH | Pituitary thyroid-stimulating hormone | Best screening test for thyroid dysfunction |
| Free T4 | Unbound thyroxine | Confirms hypo/hyperthyroidism; used with TSH |
| Free T3 | Unbound triiodothyronine | T3 toxicosis, T4→T3 conversion |
| Anti-TPO / Anti-Tg | Thyroid autoantibodies | Autoimmune thyroid disease |
| Thyroglobulin | Thyroid follicular cell protein | Tumor marker post-thyroidectomy |
| Calcitonin | C-cell hormone | Tumor marker in medullary thyroid carcinoma |
| RAIU | Iodine uptake by thyroid | Differentiate causes of hyperthyroidism |
| Morning serum cortisol | Adrenal glucocorticoid output | Screening for AI / hypercortisolism |
| 24-hr Urinary Free Cortisol | Integrated cortisol production | Screening for Cushing syndrome |
| Overnight LDDST (1 mg DST) | Pituitary-adrenal suppressibility | Screening for Cushing syndrome |
| HDDST (8 mg DST) | Pituitary vs ectopic ACTH | Localizing Cushing syndrome |
| Plasma ACTH | Pituitary corticotropin | Distinguishes primary vs secondary AI/hypercortisolism |
| ACTH Stimulation Test | Adrenal cortex reserve | Diagnosing adrenal insufficiency |
| Midnight salivary cortisol | Diurnal cortisol nadir | Screening for Cushing syndrome |
| Aldosterone + Renin (ARR) | RAAS activity | Primary hyperaldosteronism screening |
Sources:
- Harper's Illustrated Biochemistry, 32nd Ed (Adrenal Function Tests, Thyroid Function Tests)
- Quick Compendium of Clinical Pathology, 5th Ed (Thyroid chemistry)
- Henry's Clinical Diagnosis and Management by Laboratory Methods (Adrenal Function)
- Tietz Textbook of Laboratory Medicine, 7th Ed (Addison Disease, DST)
- Current Surgical Therapy, 14th Ed (Screening for Hypercortisolism)
- Costanzo Physiology, 7th Ed (HPA axis regulation)
- Goldman-Cecil Medicine (Thyroid hormone patterns)
- Tintinalli's Emergency Medicine (Thyroid function tests)
- Swanson's Family Medicine Review (DST protocols)
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