Memory tricks for hormones and how they control the body

Hormones are chemical messengers secreted into blood → act on distant target cells. Classification by solubility determines mechanism. Lipid-soluble hormones (steroids and thyroid): cross plasma membrane → bind intracellular receptors → receptor-hormone complex enters nucleus → alters gene transcription → slow but long-lasting effects. Examples: cortisol, estrogen, testosterone, thyroid hormone (T3/T4). Water-soluble hormones (peptides and catecholamines): cannot cross membrane → bind surface receptors → activate second messenger (cAMP, IP3, Ca2+) → rapid but short-lived effects. Examples: insulin, glucagon, ADH, epinephrine, GH. Eicosanoids (prostaglandins, leukotrienes): lipid-derived, local paracrine/autocrine effects. NSAIDs block their synthesis.

Anterior pituitary (adenohypophysis): controlled by hypothalamic releasing and inhibiting hormones via portal blood. Makes and releases: FSH (follicle stimulating), LH (luteinizing), ACTH (adrenocorticotropic), TSH (thyroid stimulating), GH (growth hormone), PRL (prolactin). Mnemonic: FLAT PiG. Posterior pituitary (neurohypophysis): does NOT make hormones — stores and releases hormones made by hypothalamic neurons. ADH (vasopressin) — released in response to ↑ osmolarity or ↓ BP → water retention. Oxytocin — released during labor (Ferguson reflex) and breastfeeding (milk letdown). Hypothalamus is the master controller — receives input from brain, releases regulatory hormones to control anterior pituitary, directly makes ADH and oxytocin.

Thyroid gland: two lobes connected by isthmus — largest pure endocrine gland. Follicular cells produce T3 (3 iodine atoms) and T4 (4 iodine atoms) from thyroglobulin + iodine. T4 is prohormone — converted to active T3 in peripheral tissues (by deiodinase). Effects: ↑ basal metabolic rate, ↑ heart rate and output, essential for normal brain development (deficiency in infancy = cretinism), promote bone growth and maturation, permissive for GH effects. TSH from pituitary controls production. Parafollicular (C cells) produce calcitonin: ↑ Ca2+ → calcitonin released → inhibits osteoclasts → ↓ blood Ca2+ (minor role in adults, more in children). Hypothyroidism: ↓ T3/T4, myxedema, weight gain, cold intolerance. Hyperthyroidism: ↑ T3/T4, Graves' disease (anti-TSH receptor antibodies).

Adrenal glands sit atop kidneys. Two distinct regions with different embryological origins and functions. Adrenal cortex (3 zones from outside in — "Salt, Sugar, Sex"): Zona glomerulosa → aldosterone (mineralocorticoid — Na+ retention, K+ excretion, ↑ BP). Zona fasciculata → cortisol (glucocorticoid — stress response, anti-inflammatory, ↑ blood glucose). Zona reticularis → androgens (DHEA — weak sex hormones, important in females). All controlled by ACTH (except aldosterone — controlled by RAAS and K+). Adrenal medulla: modified sympathetic ganglion — chromaffin cells secrete epinephrine (80%) and NE (20%) in response to sympathetic stimulation. Part of fight-or-flight. Pheochromocytoma: catecholamine-secreting tumor → episodic hypertension, headache, sweating.

Cortisol is the primary glucocorticoid — released from zona fasciculata in response to ACTH. HPA axis: stress → hypothalamus → CRH → anterior pituitary → ACTH → adrenal cortex → cortisol → negative feedback suppresses CRH and ACTH. Cortisol effects: ↑ blood glucose (gluconeogenesis + glycogen breakdown + ↓ glucose uptake in muscle/fat — diabetogenic), anti-inflammatory (↓ PLA2 → ↓ prostaglandins and leukotrienes, stabilizes mast cells), immunosuppressive (↓ lymphocytes, ↓ antibodies), ↑ protein catabolism (muscle breakdown), ↑ fat mobilization. Permissive effect: sensitizes blood vessels to epinephrine. Cushing's syndrome: excess cortisol → moon face, buffalo hump, central obesity, hypertension, hyperglycemia, immunosuppression, osteoporosis. Addison's disease: adrenal insufficiency → fatigue, hypotension, hyperpigmentation.

Islets of Langerhans (1-2% of pancreas): Alpha cells (20%) → glucagon. Beta cells (70%) → insulin. Delta cells (10%) → somatostatin. Insulin: released when blood glucose ↑ (eating). Effects: ↑ glucose uptake by muscle and fat (GLUT4 insertion), ↑ glycogen synthesis (liver and muscle), ↑ protein synthesis, ↑ fat storage, ↓ gluconeogenesis → overall anabolic hormone, lowers blood glucose. Glucagon: released when blood glucose ↓ (fasting). Effects: ↑ glycogenolysis (liver), ↑ gluconeogenesis → raises blood glucose — catabolic. Somatostatin: inhibits both insulin and glucagon. Also inhibits GH and GI secretions. Type 1 DM: beta cells destroyed (autoimmune) → no insulin. Type 2 DM: insulin resistance + relative insulin deficiency.

Blood calcium must stay 8.5–10.5 mg/dL — essential for nerve function, muscle contraction, clotting, bone. PTH (parathyroid hormone): from chief cells of 4 parathyroid glands. ↓ Ca2+ → PTH released → bone resorption (osteoclast stimulation), kidney Ca2+ reabsorption (DCT), kidney activates vitamin D (→ gut absorption). Net: ↑ blood Ca2+. Calcitonin: from thyroid C cells. ↑ Ca2+ → calcitonin → inhibits osteoclasts → ↓ blood Ca2+ (minor in adults). Vitamin D (calcitriol): from skin (UV) → liver (25-OH) → kidney (1,25-OH = calcitriol). Stimulates gut Ca2+ absorption (most important effect). Stimulated by PTH. Hypoparathyroidism: ↓ PTH → ↓ Ca2+ → tetany, seizures, positive Chvostek/Trousseau signs. Hyperparathyroidism: ↑ PTH → ↑ Ca2+ → kidney stones, bone pain, constipation.

GH (somatotropin) from anterior pituitary somatotrophs — most abundant pituitary hormone. Controlled by: GHRH (↑ GH), somatostatin (↓ GH), sleep (peak release during deep sleep — stage 3/4 NREM), exercise, stress, hypoglycemia all stimulate. Direct effects: ↑ protein synthesis, ↑ fat mobilization (lipolysis), ↑ blood glucose (anti-insulin = diabetogenic). Indirect effects via IGF-1 (insulin-like growth factor 1) from liver: promotes chondrocyte division → linear bone growth (before epiphyseal closure). Deficiency: dwarfism (short stature, proportionate). Excess in childhood: gigantism (tall stature). Excess in adults (after epiphyseal closure): acromegaly — enlarged hands, feet, jaw, tongue, organomegaly. Treated with somatostatin analogs (octreotide) or surgery.

Pineal gland: in brain — produces melatonin (from serotonin, dark triggers release). Controls circadian rhythm and sleep-wake cycle. ↑ at night, ↓ in light. Jet lag = circadian disruption. Thymus: in mediastinum — produces thymosin → T cell maturation and immune function. Most active in childhood, atrophies after puberty. Gonads (ovaries and testes): estrogen and progesterone (ovaries), testosterone (testes) — secondary sex characteristics, gametogenesis, reproductive function. See Reproductive Physiology for details. Heart (atria): produces ANP (atrial natriuretic peptide) when atrial walls are stretched (high blood volume) → promotes Na+ and water excretion → ↓ blood volume and pressure. Opposes RAAS. Adipose tissue: produces leptin (↑ fat stores → leptin → suppresses appetite → energy balance) and adiponectin (↑ insulin sensitivity).