General
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General endocrine principles
Negative feedback is the dominant control mechanism. Positive feedback is uncommon and self-amplifying; estrogen-triggered LH surge before ovulation is the classic example.
Peptide hormones are synthesized as preprohormones in rough ER, processed to prohormones, cleaved in Golgi/secretory granules and released by exocytosis.
Steroid hormones derive from cholesterol, are generally synthesized on demand and are not stored in large secretory granules. Major endocrine amines derive from tyrosine.
| Hormone class | Examples | Storage / transport | Receptor & response |
| Peptide/protein | Insulin, GH, ACTH, PTH, ADH | Stored in granules; usually circulate freely | Cell membrane; second messengers; rapid onset |
| Steroid | Cortisol, aldosterone, vitamin D | Made on demand; often protein-bound in plasma | Intracellular/nuclear; gene transcription; slower but longer-lasting |
| Amine | T3/T4, epinephrine, norepinephrine | Thyroid hormone stored in colloid; catecholamines in vesicles | T3/T4: nuclear and slower; catecholamines: membrane receptors and rapid |
Receptor regulation
Down-regulation: prolonged hormone exposure decreases receptor number/affinity and reduces responsiveness.
Up-regulation: increased receptor number/affinity makes the target more sensitive.
Cell signaling and second messengers
| Pathway | Sequence | High-yield examples |
| Gs → cAMP → PKA | Receptor activates adenylate cyclase; ATP becomes cAMP; PKA phosphorylates proteins | ACTH, TSH, LH, FSH, CRH, GHRH, glucagon, PTH, calcitonin, hCG, ADH-V2 |
| Gq → IP3/DAG → Ca²⁺/PKC | Phospholipase C produces IP3 and DAG; IP3 releases intracellular Ca²⁺; DAG activates PKC | GnRH, TRH, angiotensin II, ADH-V1, oxytocin, α1 receptors |
| Tyrosine kinase | Receptor dimerization/autophosphorylation activates intracellular proteins | Insulin, IGF-1 |
| Intracellular/nuclear receptor | Hormone enters cell; receptor complex regulates DNA transcription and protein synthesis | Cortisol, aldosterone, thyroid hormone, vitamin D |
| cGMP | Guanylate cyclase increases cGMP | ANP, nitric oxide |
| Ca²⁺–calmodulin | Increased intracellular calcium binds calmodulin and mediates cellular response | Ca²⁺-dependent secretion and smooth-muscle responses |
- G proteins are inactive with GDP bound and active with GTP bound. Intrinsic GTPase converts GTP back to GDP and terminates signaling.
- Phosphodiesterase breaks cAMP into 5′-AMP; inhibiting phosphodiesterase prolongs cAMP signaling.
- Water-soluble hormones usually bind membrane receptors. Lipid-soluble hormones generally enter cells and regulate transcription.
Hypothalamus and pituitary
| Structure / hormone | Key facts | MCQ association |
| Anterior pituitary | GH, prolactin, TSH, LH, FSH, ACTH; regulated by hypothalamic releasing/inhibiting hormones via portal blood | Derived from Rathke pouch/oral ectoderm |
| Posterior pituitary | Stores and releases ADH and oxytocin synthesized in hypothalamus | Neuroectoderm; does not synthesize these hormones |
| TSH, LH, FSH, hCG | Glycoproteins with the same α-subunit; different β-subunits confer specificity | Shared α-subunit |
| ACTH, MSH, β-lipotropin, β-endorphin | Derived from pro-opiomelanocortin (POMC) | POMC precursor |
Posterior pituitary
| Hormone | Origin in hypothalamus | Receptor / mechanism | Actions & regulation |
| ADH (vasopressin) | Supraoptic nucleus | V2: Gs–cAMP → AQP2 insertion in collecting-duct principal cells. V1: Gq/IP3/Ca²⁺ → vasoconstriction. | Increases water reabsorption. Increased by high plasma osmolality, volume contraction, pain, nausea, hypoglycemia, nicotine and opiates. Decreased by low osmolality, ethanol and ANP. |
| Oxytocin | Paraventricular nucleus | Gq/IP3/Ca²⁺ | Suckling causes milk ejection via myoepithelial contraction; also contracts uterus. Cervical dilation and orgasm can stimulate release. Used to induce labour and reduce postpartum bleeding. |
Pituitary traps: ADH is made in the supraoptic nucleus and oxytocin in the paraventricular nucleus; both are stored/released by the posterior pituitary. Prolactin is the anterior pituitary hormone tonically inhibited by dopamine: stalk interruption can increase prolactin while reducing other anterior pituitary output.
| Cell staining | Hormones |
| Basophils | FSH, LH, ACTH, TSH |
| Acidophils | GH and prolactin |