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Thyroid and Adrenal Gland Function Tests

MBBS Biochemistry - SGD Study Guide

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PART 1: THYROID GLAND FUNCTION TESTS

1. Thyroid Hormone Synthesis (Biochemical Basis)

1. Iodide uptake - Inorganic iodide is actively transported into the follicular cell via the sodium-iodide symporter (NIS) located in the basolateral membrane. TSH upregulates NIS activity; iodide deficiency increases pump activity, while excess iodide inhibits uptake.
2. Thyroglobulin (Tg) synthesis - Tg is a glycoprotein (~660 kDa, 2 subunits, 123 tyrosine residues) synthesized in the rough ER of thyrocytes, processed by the Golgi, and secreted into the colloid.
3. Organification - At the thyrocyte-colloid interface, iodide is oxidized to iodine by thyroid peroxidase (TPO), then incorporated into tyrosyl residues of Tg:
   • Iodide + Tyrosine → Monoiodotyrosine (MIT)
   • MIT + Iodine → Diiodotyrosine (DIT)
4. Coupling reaction (also TPO-mediated):
   • DIT + DIT → T4 (thyroxine, a prohormone)
   • MIT + DIT → T3 (triiodothyronine, the active hormone)
5. Secretion - Colloid is internalized by thyrocytes via endocytosis → lysosomal hydrolysis releases free T4 and T3 into circulation.

"Colloid represents a reservoir of thyroid hormones - humans can ingest a diet completely devoid of iodide for up to 2 months before a decline in circulating thyroid hormone levels." - Ganong's Review of Medical Physiology

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2. Transport and Metabolism of Thyroid Hormones

• T4 and T3 are highly protein-bound in serum:
  • Primarily to Thyroid Binding Globulin (TBG) and prealbumin (transthyretin)
  • Only ~0.02% of T4 and ~0.2% of T3 circulate as free (biologically active) hormones
• Peripheral conversion: ~80% of circulating T3 comes from deiodination of T4 in peripheral tissues
  • T4 → T3 (active) by 5'-deiodinase
  • T4 → Reverse T3 (rT3) (inactive metabolite)

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3. Thyroid Function Tests - Panel Overview

A. Thyroid Stimulating Hormone (TSH)

• Most sensitive and first-line test for thyroid dysfunction
• Normal range: ~0.4 - 4.0 mIU/L
• Measured by 3rd-generation chemiluminescent immunoassay (can detect <0.01 mIU/L)
• High TSH = primary hypothyroidism
• Low TSH = hyperthyroidism or secondary/tertiary hypothyroidism
• TSH alone can diagnose most outpatient thyroid disorders

A high-sensitivity TSH assay is the single best initial screening test for thyroid disease (Henry's Clinical Diagnosis and Management by Laboratory Methods).

B. Total T4 and Total T3

• Reflect both bound + free fractions
• Elevated in most hyperthyroid states; decreased in hypothyroidism
• Limitation: affected by TBG concentrations - not reliable in pregnancy, OCP use, or hypoproteinemia

C. Free T4 (FT4) and Free T3 (FT3)

• Free T4 is the biologically active fraction; gold standard measurement = equilibrium dialysis with tandem mass spectrometry (ED/MS-MS)
• Routine labs use immunoassay (automated, high-throughput)
• FT4 is unaffected by TBG changes, making it superior to total T4 in most clinical scenarios
• Important interference: Biotin supplements (common!) can cause falsely elevated FT4 or falsely low TSH in streptavidin-biotin immunoassays - patients should stop biotin 3-5 days before testing

D. T3 Resin Uptake (T3RU) / T3 Uptake Test - Historical

• An indirect measure of available TBG binding sites
• Method: Excess ¹²⁵I-labeled T3 added to serum → binds unoccupied TBG → remaining labeled T3 binds to resin
• T3RU is inversely proportional to available TBG sites and directly proportional to native T3
• High T3RU = hyperthyroidism | Low T3RU = hypothyroidism
• Free Thyroxine Index (FTI) = T3RU × Total T4 (Reference range: 5.4 - 9.7)
• Now largely replaced by direct FT4 immunoassay

E. Thyroid Antibodies

F. Thyroglobulin (Tg) as Tumor Marker

• Used to monitor differentiated thyroid cancer (papillary, follicular) after thyroidectomy + radioiodine ablation
• Rising Tg = recurrence

G. Radioactive Iodine Uptake (RAIU)

• Patient given ¹²³I or ¹³¹I orally; uptake measured at 4 and 24 hours
• High uptake: Graves' disease, toxic multinodular goitre, iodine deficiency
• Low uptake: thyroiditis (subacute, silent), exogenous thyroid hormone, iodine excess

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4. Interpretation of Thyroid Function Test Patterns

• Seen in severe illness (sepsis, major surgery, starvation)
• Low T3 (reduced peripheral conversion of T4 to T3), high rT3
• FT4 can be low in severe cases
• TSH usually normal - distinguishes it from true hypothyroidism

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5. Step-wise Approach to Thyroid Testing (SGD Algorithm)

Start: Measure TSH
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   TSH normal → Euthyroid (no further testing usually needed)
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   TSH low → Measure FT4 + FT3
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          FT4/FT3 high → Primary hyperthyroidism
          FT4/FT3 normal → Subclinical hyperthyroidism
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   TSH high → Measure FT4
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          FT4 low → Primary hypothyroidism
          FT4 normal → Subclinical hypothyroidism
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   TSH low + FT4 low → Secondary/tertiary hypothyroidism
                        (pituitary/hypothalamic cause)

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PART 2: ADRENAL GLAND FUNCTION TESTS

1. Adrenal Cortex - Zones and Products (Biochemical Basis)

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2. HPA Axis and Cortisol Regulation

Hypothalamus → CRH (+ AVP/ADH)
                 ↓
Anterior Pituitary → ACTH
                 ↓
Adrenal Cortex (Zona Fasciculata) → Cortisol
                 ↓
       Negative feedback on both
       hypothalamus and pituitary

• Cortisol secretion follows a circadian rhythm - peak at 8 AM, nadir at midnight
• StAR (Steroidogenic Acute Regulatory Protein) is the rate-limiting step: shuttles cholesterol across the inner mitochondrial membrane to begin steroidogenesis

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3. Key Adrenal Function Tests

A. Tests for Glucocorticoid Excess (Cushing's Syndrome)

• Measure plasma ACTH:
  • ACTH elevated or normal → ACTH-dependent (pituitary adenoma = Cushing's disease, or ectopic ACTH)
  • ACTH suppressed/undetectable → ACTH-independent (adrenal adenoma/carcinoma)

• High-dose DST (8 mg overnight): Suppression in Cushing's disease (pituitary source); NO suppression in ectopic ACTH
• CRH stimulation test: Exaggerated ACTH/cortisol response in pituitary Cushing's; blunted in ectopic
• Inferior petrosal sinus sampling (IPSS): Gold standard for confirming pituitary vs. ectopic ACTH

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B. Tests for Glucocorticoid Deficiency (Addison's Disease / Adrenal Insufficiency)

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C. Tests for Mineralocorticoid Excess (Conn's Syndrome / Primary Hyperaldosteronism)

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D. Tests for Adrenal Medulla - Pheochromocytoma

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E. Tests for Adrenal Androgens

• DHEA-S (dehydroepiandrosterone sulfate): Marker of adrenal androgen production; elevated in adrenal tumors and CAH
• 17-OH Progesterone (17-OHP): Key marker for 21-hydroxylase deficiency (most common form of CAH)
  • Elevated 17-OHP + elevated urinary pregnanetriol = CAH diagnosis
• Urinary 17-ketosteroids (17-KS): Reflect androgen metabolites; elevated in adrenal androgen excess

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4. Congenital Adrenal Hyperplasia (CAH) - Biochemical Features

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5. Summary Table - Adrenal Diseases and Key Tests

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SGD Discussion Questions

1. Why is TSH the most sensitive test for thyroid dysfunction rather than T3 or T4? (Hint: Think logarithmic relationship between TSH and free T4)
2. A pregnant woman has high total T4 but normal FT4 and is asymptomatic. How do you explain this? Which test is more reliable in pregnancy?
3. Explain why the T3RU is inversely proportional to TBG but directly proportional to serum T3.
4. A patient presents with low cortisol and high ACTH. Localize the lesion. What test would you perform next?
5. Why does 21-hydroxylase deficiency cause both salt-wasting and virilization? Trace the steroid pathway.
6. What is the biochemical basis of hypertension in Conn's syndrome? How does the aldosterone:renin ratio help diagnose it?
7. Differentiate Cushing's syndrome vs. Cushing's disease biochemically.

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• Quick Compendium of Clinical Pathology, 5th Edition - Thyroid function test patterns
• Henry's Clinical Diagnosis and Management by Laboratory Methods - Thyroid synthesis, adrenal zones, CAH
• Ganong's Review of Medical Physiology, 26th Edition - Thyroid hormone synthesis and secretion
• Basic Medical Biochemistry: A Clinical Approach, 6th Edition - Adrenal steroidogenesis