Physiology — Endocrine & Reproductive — USMLE Step 1 Notes
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Hypothalamic-pituitary axis
- The hypothalamus releases releasing/inhibiting hormones to the anterior pituitary (via the portal system), which secretes ACTH, TSH, FSH, LH, GH and prolactin.
- The posterior pituitary stores and releases ADH and oxytocin made in the hypothalamus.
- Most axes run on negative feedback: the end-organ hormone suppresses the hypothalamus and pituitary.
Thyroid
- TRH → TSH → thyroid makes T4 (and some T3); peripheral deiodinase converts T4 to the more active T3.
- Thyroid hormone raises basal metabolic rate, heat production and the number/sensitivity of beta-adrenergic receptors.
Adrenal gland
- Cortex (GFR outside-in): zona Glomerulosa → aldosterone; Fasciculata → cortisol; Reticularis → androgens.
- Cortisol raises blood glucose (gluconeogenesis), is permissive for catecholamines, and is anti-inflammatory/immunosuppressive; it follows a diurnal rhythm.
- Medulla secretes catecholamines (adrenaline > noradrenaline) — a modified sympathetic ganglion.
Glucose homeostasis
- Insulin (beta cells) is the only hormone that lowers glucose: promotes uptake (GLUT4 in muscle/fat), glycogenesis, lipogenesis.
- Glucagon (alpha cells) raises glucose via glycogenolysis and gluconeogenesis; adrenaline, cortisol and GH are the other counter-regulatory hormones.
Calcium & bone
- PTH raises calcium: bone resorption, distal renal Ca reabsorption, phosphate excretion, and activation of vitamin D.
- Active vitamin D (calcitriol) raises gut absorption of calcium and phosphate; calcitonin (minor) lowers calcium.
Growth hormone
- GH acts directly and via hepatic IGF-1; it promotes linear growth, protein synthesis and lipolysis, and is diabetogenic (raises glucose).
Reproductive axis
- GnRH (pulsatile) → FSH and LH. In women the mid-cycle oestrogen surge flips to positive feedback, triggering the LH surge and ovulation; the corpus luteum then makes progesterone.
- In men, LH drives Leydig-cell testosterone; FSH supports Sertoli cells and spermatogenesis (inhibin feeds back on FSH).
- In pregnancy, hCG sustains the corpus luteum until the placenta takes over hormone production.
Written by StepSherpa in our own words, grounded in Human Physiology (Wikibooks), used under CC BY-SA 3.0.
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