Endocrinology

Hormone System

The endocrine glands and the chemical messengers that coordinate the body

The endocrine system is a network of ductless glands that secrete hormones into the bloodstream to coordinate metabolism, growth, reproduction, and stress responses. Major glands — hypothalamus, pituitary, pineal, thyroid, parathyroid, adrenals, pancreas (islets), gonads (ovaries, testes). Hormones come in three classes — peptide (insulin, growth hormone, oxytocin), steroid (cortisol, estrogens, androgens, aldosterone, vitamin D), and amine (thyroid hormones, catecholamines). Peptides bind cell-surface receptors via second messengers (cAMP, IP3); steroids and thyroid hormones cross membranes to bind nuclear receptors regulating transcription. Disorders include hyper- and hyposecretion, end-organ resistance, and tumors. The hormone system overlaps with the nervous system at the hypothalamus and adrenal medulla.

  • Major glandsHypothalamus, pituitary, thyroid, parathyroid, adrenals, pancreas, gonads
  • Hormone classesPeptide, steroid, amine
  • Master glandAnterior pituitary (under hypothalamus)
  • Largest endocrine organThyroid by mass; pancreas by hormone diversity
  • Half-livesPeptide minutes; steroid hours; T4 ~7 days
  • Receptor locationSurface (peptides); nuclear (steroids, thyroid)

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Why endocrine matters

  • Diabetes care. Most common endocrine disorder.
  • Thyroid disease. Affects 5-10% of population; common cause of fatigue, weight change.
  • Osteoporosis. PTH, vitamin D, sex steroids interact.
  • Reproductive health. Infertility, menopause, contraception.
  • Hypertension causes. Aldosterone, pheochromocytoma, Cushing screened in resistant cases.
  • Critical illness. Adrenal insufficiency masquerades as refractory shock.
  • Pediatric growth. GH deficiency, Turner syndrome, congenital hypothyroidism.

Common errors

  • Random cortisol for adrenal insufficiency. Use AM cortisol and ACTH-stim.
  • Random GH levels. Pulsatile; use IGF-1 and suppression tests.
  • Treating subclinical hypothyroidism reflexively. Treat if TSH >10, symptomatic, pregnant.
  • Beta blocker before alpha in pheochromocytoma. Unopposed alpha causes hypertensive crisis.
  • Missing secondary hypertension. Young, resistant, hypokalemic patients need workup.
  • Ignoring assay interference. Biotin, antibodies skew immunoassays.

Frequently asked questions

What does the pituitary do?

Anterior pituitary releases six tropic hormones — TSH (thyroid), ACTH (adrenal cortex), LH/FSH (gonads), GH (growth, IGF-1), prolactin (lactation). Posterior pituitary stores and releases ADH (vasopressin, water retention) and oxytocin (uterine contraction, milk letdown), made in hypothalamus. Tumors — prolactinoma (most common; treat with cabergoline or bromocriptine), GH-secreting (acromegaly; surgery, octreotide, pegvisomant), ACTH-secreting (Cushing disease), nonfunctional macroadenomas (mass effect, bitemporal hemianopsia from optic chiasm compression).

How does the thyroid work?

Follicular cells take up iodide, organify it on tyrosine residues of thyroglobulin, couple to form T3 (triiodothyronine, active) and T4 (thyroxine, prohormone). Daily output 80% T4, 20% T3; peripheral deiodinases convert T4 to T3 in target tissues. Hyperthyroidism — Graves (TSI antibodies, ophthalmopathy), toxic adenoma, multinodular goiter; treat with methimazole, beta blocker, radioactive iodine, surgery. Hypothyroidism — Hashimoto, iodine deficiency, post-ablation; treat with levothyroxine 1.6 µg/kg/day.

What does the adrenal gland do?

Cortex makes three layers of steroids. Zona glomerulosa — aldosterone (mineralocorticoid; Na retention, K excretion; under RAAS). Zona fasciculata — cortisol (glucocorticoid; gluconeogenesis, immunosuppression; under ACTH). Zona reticularis — DHEA, androstenedione (sex steroid precursors). Medulla — chromaffin cells make epinephrine and norepinephrine (catecholamines). Pheochromocytoma — adrenal medulla tumor with paroxysmal hypertension, headache, palpitations, sweating; treat with alpha blocker before beta blocker, then surgery.

How do islet cells work?

Pancreatic islets of Langerhans contain alpha (glucagon — raises glucose), beta (insulin — lowers glucose), delta (somatostatin — inhibits both), PP (pancreatic polypeptide), epsilon (ghrelin) cells. Insulin promotes glucose uptake by GLUT4 in muscle and adipose, glycogen synthesis, lipogenesis. Glucagon promotes hepatic gluconeogenesis and glycogenolysis. Type 1 diabetes — autoimmune beta-cell destruction. Type 2 — insulin resistance plus progressive beta failure. GLP-1 from L cells stimulates insulin and inhibits glucagon — basis for semaglutide, liraglutide.

What about parathyroid and calcium?

Four parathyroid glands behind thyroid sense ionized calcium via CaSR. Falling Ca triggers PTH release. PTH increases renal Ca reabsorption and phosphate excretion, activates 1-alpha-hydroxylase to make calcitriol (active vitamin D, increases gut Ca absorption), and stimulates osteoclasts via RANKL. Primary hyperparathyroidism — adenoma, hypercalcemia, bone, stones, abdominal moans, psychic groans; surgery if symptomatic or by criteria. Secondary — CKD with low calcitriol and high phosphate. Tertiary — autonomous after long secondary.

What are gonadal hormones?

Ovary makes estradiol (estrogen, follicle), progesterone (corpus luteum), small testosterone. Estradiol drives follicular phase, endometrial proliferation; LH surge triggers ovulation; progesterone dominates luteal phase. Testes Leydig cells make testosterone under LH; Sertoli cells support spermatogenesis under FSH. Disorders — PCOS (anovulation, hyperandrogenism, insulin resistance), hypogonadism (Klinefelter, Kallmann, testicular failure), congenital adrenal hyperplasia (21-hydroxylase deficiency).

How are hormones measured?

Random levels often unhelpful for hormones with circadian or pulsatile release (cortisol, GH, LH, ACTH). Tests include — TSH and free T4 for thyroid (random fine because of slow change). Cortisol — 8 AM and midnight salivary, 24-hour urine, dex suppression. GH — IGF-1 surrogate plus oral glucose suppression. ACTH-stim test for adrenal insufficiency. PTH plus calcium and phosphate. Insulin and C-peptide with fasting glucose for hypoglycemia workup. Always consider assay interference (biotin, heterophile antibodies).