Assessment of Endocrine Function

Foundations & Methods - Assay Arsenal

• Immunoassays: Cornerstone for hormone quantification.
  • Competitive (e.g., RIA): Labeled hormone competes with patient hormone for antibody binding. Signal inversely proportional to analyte.
  • Non-competitive/Sandwich (e.g., IRMA, ELISA, CLIA): Analyte "sandwiched" between two antibodies. Signal directly proportional. Generally higher sensitivity.
• Chromatography (HPLC, GC-MS): Separates & quantifies structurally similar hormones (e.g., steroids, catecholamines).
• Bioassays: Measure biological effect; mainly research.
• Dynamic Function Tests (DFTs): Assess endocrine gland responsiveness.
  • Stimulation tests: Evaluate reserve capacity (e.g., ACTH stimulation for adrenal insufficiency).
  • Suppression tests: Check feedback integrity & autonomy (e.g., Dexamethasone suppression for Cushing's syndrome).

⭐ ELISA is widely used due to its versatility, safety (no radioisotopes), and suitability for automation.

Pituitary Puzzles - Master Gland Checkup

• Strategy: Clinical exam, baseline hormones, dynamic tests, imaging.
• Baseline Hormones:
  • Anterior: GH (via IGF-1), PRL, ACTH (8 AM cortisol), TSH (fT4), LH/FSH (sex steroids).
  • Posterior: ADH (serum/urine osmolality).
• Dynamic Function Tests:
  • Stimulation (hypofunction):
    • ITT: GH & ACTH. 📌 "Insulin STRESSES Pituitary to ACT & GROW."
    • Glucagon stim: GH & ACTH alt.
  • Suppression (hyperfunction):
    • OGTT: GH suppression (Acromegaly).
    • DST: ACTH (Cushing's).
• Imaging: MRI (sella) primary.

⭐ The Insulin Tolerance Test (ITT) is gold standard for GH & ACTH reserve; hypoglycemia (glucose < 2.2 mmol/L or 40 mg/dL) is key for stimulation.

Thyroid & Adrenal Tales - Axis Investigations

Thyroid Axis (HPT):

• TSH: Best initial.
  • ↑ TSH, ↓ Free T4: Primary hypothyroidism.
  • ↓ TSH, ↑ Free T4/T3: Primary hyperthyroidism.
  • Normal/↓ TSH, ↓ Free T4: Central hypothyroidism.
• Antibodies: Anti-TPO Ab (Hashimoto's), TRAb (Graves').
• RAIU Scan: Hot (active) vs. Cold (inactive, ↑ Ca risk) nodules.

Adrenal Axis (HPA):

• Cushing's Syndrome:
  • Screening: 24-hr UFC, Late-night salivary cortisol, 1mg ONDST.
  • Localization: Plasma ACTH, 8mg HDDST, CRH stimulation.
• Adrenal Insufficiency:
  • Basal: Morning cortisol, ACTH.
  • Dynamic: ACTH stimulation (Synacthen 250µg); subnormal rise → Addison's.
• Pheochromocytoma: Plasma/Urinary metanephrines & VMA.
• Hyperaldosteronism: Aldosterone-Renin Ratio (ARR). 📌 DST: Low dose screens, High dose differentiates.

⭐ Differentiating Cushing's Disease (pituitary) from ectopic ACTH: HDDST suppresses cortisol in most Cushing's Disease, not ectopic ACTH.

Sugar & Bone Balance - Metabolic Markers

• Glucose Homeostasis Markers
  • Fasting Plasma Glucose (FPG): Normal <100; Diabetes ≥126 mg/dL.
  • Post-Prandial (2-hr OGTT): Normal <140; Diabetes ≥200 mg/dL.
  • HbA1c: Reflects ~3 months glucose. Normal <5.7%; Diabetes ≥6.5%.
  • C-peptide: Endogenous insulin; ↓ T1DM.
  • Autoantibodies (GAD65, IAA): For T1DM.
• Calcium & Bone Metabolism Markers
  • Serum Calcium: Total (8.5-10.5 mg/dL), Ionized. Correct for albumin.
  • Serum Phosphate: (2.5-4.5 mg/dL).
  • PTH: Regulates Ca & PO₄.
  • Vitamin D: 25(OH)D (status, <20 ng/mL deficiency); 1,25(OH)₂D (active).
  • Alkaline Phosphatase (ALP): Bone-specific, osteoblast activity.
  • Bone Turnover Markers (BTMs):
    • Resorption: CTX, NTX.
    • Formation: P1NP, Osteocalcin.

⭐ HbA1c reliability is ↓ with altered RBC lifespan (e.g., hemolytic anemia, CKD).

High‑Yield Points - ⚡ Biggest Takeaways

• Dynamic function tests (stimulation/suppression) are superior to basal levels for assessing endocrine reserve and autonomy.
• Free hormone assays (e.g., free T4) are generally preferred over total hormone levels, minimizing binding protein effects.
• 24-hour urine collections (e.g., for cortisol, metanephrines) account for variable secretion patterns like pulsatile or diurnal rhythms.
• Imaging (MRI, CT, USG) is for localization after biochemical diagnosis of endocrine dysfunction, not for screening.
• Specific autoantibody tests are crucial for diagnosing autoimmune endocrinopathies (e.g., Graves' disease, Hashimoto's thyroiditis).