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PY8.1-7 | Endocrine Physiology — Part 1
CLINICAL SCENARIO
Kavya, a 19-year-old MBBS first-year student from Chennai, noticed she had lost 8 kg in 3 months despite eating more than usual. She was sweating through lectures even in an air-conditioned hall, her heart seemed to race continuously (HR 118/min at rest), and she was irritable with everyone. Her friends joked that she was 'stressed about exams.' When she finally saw a doctor, a blood test revealed: TSH < 0.01 mIU/L (normal 0.5–4.5), Free T4 = 4.8 ng/dL (normal 0.8–1.8), Free T3 = 12 pg/mL (normal 2.3–4.2). A radioiodine uptake scan showed diffuse increased uptake.
Diagnosis: Graves' disease — autoimmune hyperthyroidism. The excess thyroid hormone is driving up her metabolic rate, heart rate, and thermogenesis. Every symptom maps directly to thyroid hormone physiology.
By the end of this module, you will understand why these blood results mean what they mean, why the scan looks the way it does, and what the treatment options are based on the physiology.
WHY THIS MATTERS
Endocrine disorders are among the most common presentations in primary care in India:
- Diabetes mellitus: 101 million Indians (2023 IDF data) — the highest absolute number in the world. Understanding glucose homeostasis is not optional for an Indian doctor.
- Thyroid disorders: Hypothyroidism affects 1 in 10 women in India (largely iodine-deficiency goitre in some regions, autoimmune in urban areas). Hypothyroidism is the most commonly missed diagnosis in female patients.
- Calcium disorders: Vitamin D deficiency is near-universal in India (due to indoor work, skin pigmentation, dietary deficiency). Hypocalcaemia causes tetany — a treatable emergency.
- Adrenal emergencies: Addisonian crisis (adrenal insufficiency) is rare but lethal if missed — and the workup requires understanding cortisol physiology.
Every endocrine drug in the pharmacopoeia (insulin, metformin, levothyroxine, prednisolone, bisphosphonates) requires understanding this physiology.
RECALL
Endocrine physiology builds on concepts from multiple subjects:
From Biochemistry (BI):
- Steroid hormones are derived from cholesterol — you studied lipid chemistry in BI4
- Glucose metabolism (glycolysis, gluconeogenesis, glycogen synthesis) — studied in BI3; insulin and glucagon regulate these pathways
- Iodine metabolism and thyroid hormone synthesis require understanding of amino acid chemistry (tyrosine)
- Vitamin D activation: cholecalciferol → 25-OH in liver → 1,25-(OH)₂ in kidney (requires knowing the relevant enzymes)
From Anatomy (AN):
- Pituitary gland: situated in sella turcica, connected to hypothalamus via portal blood supply
- Thyroid gland: two lobes + isthmus, parafollicular C cells (calcitonin) alongside follicular cells (T3/T4)
- Adrenal gland: cortex (3 zones) + medulla — different embryological origin (cortex = mesoderm, medulla = neural crest)
- Islets of Langerhans: α cells (glucagon), β cells (insulin), δ cells (somatostatin)
This module gives function to all that structure.
The Hypothalamus and Pituitary: The Master Control System
Hypothalamic-Pituitary Hormone Pairs
| Hypothalamic Hormone | Effect | Pituitary Hormone | Target Organ |
|---|---|---|---|
| GnRH (pulsatile) | Stimulates | FSH + LH | Gonads (ovaries/testes) |
| TRH | Stimulates | TSH (also prolactin) | Thyroid gland |
| CRH | Stimulates | ACTH | Adrenal cortex |
| GHRH | Stimulates | GH | Liver (IGF-1), bones, tissues |
| Somatostatin | Inhibits | GH (and TSH) | — |
| Dopamine | Inhibits | Prolactin | — (Prolactin is unique: tonically inhibited) |
Hypothalamic-Pituitary Hormone Pairs
Figure: The Hypothalamus and Pituitary: The Master Control System
| Hypothalamic Hormone | Effect | Pituitary Hormone | Target Organ |
|---|---|---|---|
| GnRH (pulsatile) | Stimulates | FSH + LH | Gonads (ovaries/testes) |
| TRH | Stimulates | TSH (also prolactin) | Thyroid gland |
| CRH | Stimulates | ACTH | Adrenal cortex |
| GHRH | Stimulates | GH | Liver (IGF-1), bones, tissues |
| Somatostatin | Inhibits | GH (and TSH) | — |
| Dopamine | Inhibits | Prolactin | — (Prolactin is unique: tonically inhibited) |
Hypothalamic-Pituitary Hormone Pairs
| Hypothalamic Hormone | Effect | Pituitary Hormone | Target Organ |
|---|---|---|---|
| GnRH (pulsatile) | Stimulates | FSH + LH | Gonads (ovaries/testes) |
| TRH | Stimulates | TSH (also prolactin) | Thyroid gland |
| CRH | Stimulates | ACTH | Adrenal cortex |
| GHRH | Stimulates | GH | Liver (IGF-1), bones, tissues |
| Somatostatin | Inhibits | GH (and TSH) | — |
| Dopamine | Inhibits | Prolactin | — (Prolactin is unique: tonically inhibited) |
The hypothalamus is the interface between the nervous system and the endocrine system. It receives inputs from the cortex (emotions, light-dark cycle, stress) and outputs via two mechanisms:
Figure: The Hypothalamus and Pituitary: The Master Control System
1. Hypothalamo-pituitary portal blood supply (for anterior pituitary):
Hypothalamic releasing/inhibiting hormones travel in short portal vessels (NOT systemic blood) to the anterior pituitary. Key pairs:
| Hypothalamic hormone | Effect on pituitary |
|---|---|
| GnRH (pulsatile) | ↑ FSH + LH |
| TRH | ↑ TSH (also ↑ prolactin) |
| CRH | ↑ ACTH |
| GHRH | ↑ GH |
| Somatostatin | ↓ GH (and TSH) |
| Dopamine | ↓ Prolactin |
2. Direct neuronal projections to posterior pituitary:
Oxytocin and ADH (vasopressin) are synthesised in hypothalamic nuclei (paraventricular and supraoptic), travel down axons, and are stored and released from the posterior pituitary.
Anterior pituitary hormones (mnemonic: FLAT PiG):
FSH, LH, ACTH, TSH, Prolactin, iGF-1 mediator (GH).
All anterior pituitary hormones are regulated by negative feedback from the target organ hormone. Example: T4 inhibits both TRH (hypothalamus) and TSH (pituitary) — this is why a TSH < 0.01 in Kavya means very high T4 has suppressed TSH.
Growth hormone (GH): Released in pulses, peaks during slow-wave sleep. Primary targets: liver → IGF-1 (mediates growth effects) and peripheral tissues (lipolysis, anti-insulin effects). Regulated by GHRH (↑), somatostatin (↓), hypoglycaemia (↑), sleep (↑).
Posterior Pituitary: ADH and Oxytocin
Diabetes Insipidus vs SIADH
| Feature | Central DI | Nephrogenic DI | SIADH |
|---|---|---|---|
| ADH level | Low/absent | Normal or high | Inappropriately high |
| Cause | Pituitary surgery, head trauma, tumour | Lithium, hypercalcaemia, genetic | CNS disease, ectopic production (lung cancer), drugs |
| Urine output | High (up to 20 L/day), dilute | High, dilute | Low, concentrated |
| Plasma osmolality | High | High | Low (dilutional) |
| Serum Na+ | High (hypernatraemia) | High | Low (hyponatraemia) |
| Treatment | Desmopressin (DDAVP) | Treat underlying cause, thiazides | Fluid restriction, tolvaptan (V2 antagonist) |
Diabetes Insipidus vs SIADH
Figure: Posterior Pituitary: ADH and Oxytocin
| Feature | Central DI | Nephrogenic DI | SIADH |
|---|---|---|---|
| ADH level | Low/absent | Normal or high | Inappropriately high |
| Cause | Pituitary surgery, head trauma, tumour | Lithium, hypercalcaemia, genetic | CNS disease, ectopic production (lung cancer), drugs |
| Urine output | High (up to 20 L/day), dilute | High, dilute | Low, concentrated |
| Plasma osmolality | High | High | Low (dilutional) |
| Serum Na+ | High (hypernatraemia) | High | Low (hyponatraemia) |
| Treatment | Desmopressin (DDAVP) | Treat underlying cause, thiazides | Fluid restriction, tolvaptan (V2 antagonist) |
Diabetes Insipidus vs SIADH
| Feature | Central DI | Nephrogenic DI | SIADH |
|---|---|---|---|
| ADH level | Low/absent | Normal or high | Inappropriately high |
| Cause | Pituitary surgery, head trauma, tumour | Lithium, hypercalcaemia, genetic | CNS disease, ectopic production (lung cancer), drugs |
| Urine output | High (up to 20 L/day), dilute | High, dilute | Low, concentrated |
| Plasma osmolality | High | High | Low (dilutional) |
| Serum Na+ | High (hypernatraemia) | High | Low (hyponatraemia) |
| Treatment | Desmopressin (DDAVP) | Treat underlying cause, thiazides | Fluid restriction, tolvaptan (V2 antagonist) |
ADH (Antidiuretic Hormone / Vasopressin):
Synthesised in the supraoptic nucleus. Released by:
- ↑ plasma osmolality (sensed by osmoreceptors in the hypothalamus) — most potent stimulus
- ↓ blood volume or pressure (sensed by baroreceptors in atria and carotid sinus)
- Pain, nausea, stress, nicotine
- Alcohol inhibits ADH → diuresis (explaining why alcohol causes dehydration)
Figure: Posterior Pituitary: ADH and Oxytocin
Actions:
- V2 receptors in collecting duct → inserts aquaporin-2 → water reabsorption → concentrated urine (already studied in renal module)
- V1 receptors in vascular smooth muscle → vasoconstriction (used in septic shock as vasopressin infusion)
SIADH (Syndrome of Inappropriate ADH): ADH secreted despite low plasma osmolality → water retention → dilutional hyponatraemia. Causes: CNS disease, lung cancer (ectopic production), drugs (carbamazepine, SSRIs). Treatment: fluid restriction; tolvaptan (V2 receptor antagonist).
Oxytocin:
Synthesised in the paraventricular nucleus. Released by:
- Cervical distension during labour (Ferguson reflex — a positive feedback loop!)
- Suckling by the infant (milk ejection reflex)
Actions: uterine contractions (labour), milk ejection (myoepithelial cells of breast). Also has CNS roles: bonding, trust, social recognition.
Oxytocin drip is used to induce or augment labour and to prevent postpartum haemorrhage — a common obstetric intervention you will use.
CLINICAL PEARL
TSH responds to tiny changes in free T4 with a logarithmic (not linear) amplification. A 2-fold change in free T4 causes a 100-fold change in TSH. This means TSH is an exquisitely sensitive indicator of thyroid status — it detects early (subclinical) hypo- or hyperthyroidism before symptoms appear. In Kavya's case, TSH < 0.01 tells you that her pituitary is being completely suppressed by very high T4/T3. Lesson: Always check TSH first when screening for thyroid disease. Order free T4 only if TSH is abnormal.
Thyroid Hormone: Synthesis and Regulation
Thyroid follicular cells produce T4 (thyroxine) and T3 (tri-iodothyronine). T4 is the main secretory product (90%), but T3 is 3–4× more potent and is the active form at target cells (T4 is converted to T3 by peripheral deiodinases).
Figure: Thyroid Hormone: Synthesis and Regulation
Synthesis (the iodine trap — key for understanding radioiodine therapy):
1. Iodide trapping: Follicular cells actively transport I⁻ from blood against a concentration gradient (Na-I symporter, NIS). TSH upregulates NIS. This is why radioiodine (¹³¹I) accumulates in thyroid — used for both imaging and treatment.
2. Oxidation: I⁻ → I₂ (by thyroid peroxidase, TPO)
3. Organification: I₂ attaches to tyrosine residues on thyroglobulin (a large protein stored in follicular colloid) → forms MIT (mono-iodotyrosine) and DIT (di-iodotyrosine)
4. Coupling: MIT + DIT → T3; DIT + DIT → T4
5. Secretion: TSH → endocytosis of colloid → lysosomal hydrolysis of thyroglobulin → T4 and T3 released
Antithyroid drugs: Propylthiouracil (PTU) and carbimazole block TPO → inhibit organification and coupling. PTU also blocks peripheral T4 → T3 conversion.
Actions of thyroid hormones (T3/T4):
- ↑ Basal metabolic rate (BMR) — upregulate Na-K-ATPase → more ATP consumption → more heat
- ↑ Heart rate and cardiac output (upregulate β1 receptors)
- ↑ Bone growth and CNS maturation (essential for foetal brain development)
- Permissive effects: required for GH and catecholamine actions
- ↑ Protein synthesis (anabolic in physiological doses), catabolic in excess