Endocrine System — revision sheet

The endocrine system is a network of ductless glands and scattered hormone-secreting cells that release chemical messengers into the blood. Working alongside the nervous system, it sets the slow, sustained background of metabolism, growth, fluid and electrolyte balance, stress responses and reproduction, mostly through hormone cascades kept stable by negative feedback.

Pituitary gland: ≈1 cm across; ≈0.5–0.6 g (enlarges in pregnancy)Thyroid gland: ≈15–25 g; isthmus over tracheal rings 2–4Adrenal gland: ≈4–5 g each; cortex ≈80–90 %, medulla ≈10–20 %Pancreatic islets: ≈1 million (1–2 % of pancreatic mass); β cells ≈60–70 %Fasting plasma glucose: 3.9–5.5 mmol/L (70–99 mg/dL)Serum calcium (total / ionised): 2.2–2.6 mmol/L / 1.1–1.3 mmol/LTSH (adult): ≈0.4–4.0 mU/LFree T4 / free T3 fraction: ≈0.03 % / ≈0.3 % of total (rest protein-bound)08:00 serum cortisol: ≈140–690 nmol/L (≈5–25 µg/dL); nadir near midnightPlasma osmolality: 275–295 mOsm/kg (ADH release threshold ≈280–285)Half-lives: Insulin ≈5 min · cortisol ≈60–90 min · T3 ≈1 day · T4 ≈7 daysDiabetes thresholds: Fasting glucose ≥7.0 mmol/L · HbA1c ≥48 mmol/mol (6.5 %) · 2-h OGTT ≥11.1 mmol/L

Hypothalamus & pituitary

Hypothalamus Brain region: The small ventral part of the diencephalon (≈4 g) that links the nervous and endocrine systems, controlling the pituitary, the autonomic nervous system and homeostatic drives such as thirst, hunger and temperature.
Supraoptic nucleus Brain region: A paired cluster of large (magnocellular) neurosecretory neurons straddling the optic tract that synthesises mainly antidiuretic hormone (vasopressin) for release from the posterior pituitary.
Paraventricular nucleus Brain region: A wing-shaped nucleus in the wall of the third ventricle that contains magnocellular oxytocin/ADH neurons, parvocellular CRH and TRH neurons, and pre-autonomic neurons — the hub of the stress response.
Arcuate nucleus Brain region: A crescent of small neurons around the base of the third ventricle above the median eminence that secretes GHRH, dopamine and (with the preoptic area) GnRH, and integrates leptin, insulin and ghrelin signals to control appetite.
Suprachiasmatic nucleus Brain region: A pair of tiny nuclei (≈10 000 neurons each) just above the optic chiasm that act as the body's master circadian clock, entrained by light and driving melatonin and cortisol rhythms.
Pituitary stalk and median eminence Organ part: The funnel-shaped connection between the hypothalamus and pituitary, carrying neurosecretory axons to the posterior lobe and the portal vessels that link the median eminence to the anterior lobe.
Pituitary gland Gland: A pea-sized gland (≈1 cm, ≈0.5–0.6 g) sitting in the sella turcica beneath the hypothalamus; its anterior lobe secretes six tropic hormones and its posterior lobe releases ADH and oxytocin.
Anterior pituitary (adenohypophysis) Organ part: The glandular, epithelial lobe of the pituitary that secretes six tropic hormones — GH, prolactin, TSH, ACTH, FSH and LH — under control of hypothalamic hormones delivered by portal blood.
Posterior pituitary (neurohypophysis) Organ part: The neural lobe of the pituitary, made of axon terminals of hypothalamic neurons, which stores and releases antidiuretic hormone and oxytocin directly into the blood.

Pineal gland

Pineal gland Gland: A pine-cone-shaped midline gland (≈5–8 mm) in the epithalamus that secretes melatonin at night, translating the light–dark cycle into a hormonal signal of darkness.
Melatonin Hormone: An indoleamine derived from tryptophan and secreted by the pineal gland at night under the control of the suprachiasmatic clock; it signals darkness, promotes sleep and helps entrain circadian rhythms.

Thyroid & parathyroid glands

Thyroid gland Gland: A butterfly-shaped gland (≈15–25 g) in the anterior neck that stores iodinated thyroglobulin in follicles and secretes thyroxine (T4) and triiodothyronine (T3), which set metabolic rate, plus calcitonin from its C cells.
Thyroid follicle Tissue: The spherical functional unit of the thyroid — a single layer of follicular cells (thyrocytes) surrounding a lumen of colloid — where thyroid hormones are made on the scaffold of thyroglobulin.
Parafollicular cell (C cell) Cell: Pale neuroendocrine cells scattered between and within thyroid follicles that secrete calcitonin in response to high blood calcium; they give rise to medullary thyroid carcinoma.
Parathyroid glands Gland: Usually four lentil-sized glands (≈30–50 mg each) on the posterior surface of the thyroid whose chief cells secrete parathyroid hormone, the minute-to-minute regulator of blood calcium.

Adrenal glands

Adrenal (suprarenal) gland Gland: Paired retroperitoneal glands (≈4–5 g each) capping the upper poles of the kidneys, each made of an outer steroid-secreting cortex and an inner catecholamine-secreting medulla.
Adrenal cortex Organ part: The outer steroid-secreting part of the adrenal gland, organised into three zones — glomerulosa, fasciculata and reticularis — that produce aldosterone, cortisol and adrenal androgens ("salt, sugar, sex").
Adrenal medulla Organ part: The core of the adrenal gland — a modified sympathetic ganglion of chromaffin cells that secrete epinephrine (≈80 %) and norepinephrine (≈20 %) into the blood during stress.

Pancreatic islets

Pancreatic islets (islets of Langerhans) Tissue: About a million microscopic clusters of endocrine cells scattered through the exocrine pancreas (1–2 % of its mass) that secrete insulin, glucagon, somatostatin and pancreatic polypeptide to control fuel metabolism.

Hypothalamic hormones

Thyrotropin-releasing hormone (TRH) Hormone: A tripeptide from parvocellular paraventricular neurons that stimulates thyrotrophs to release TSH (and lactotrophs to release prolactin) — the top of the hypothalamic–pituitary–thyroid axis.
Corticotropin-releasing hormone (CRH) Hormone: A 41-amino-acid peptide from paraventricular neurons that drives ACTH release and orchestrates the endocrine, autonomic and behavioural responses to stress.
Gonadotropin-releasing hormone (GnRH) Hormone: A decapeptide released in pulses (every ≈60–120 min) from hypothalamic neurons that stimulates LH and FSH secretion; pulsatility is essential — continuous GnRH shuts the axis down.
Growth hormone-releasing hormone (GHRH) Hormone: A 44-amino-acid peptide from arcuate neurons that stimulates GH synthesis, secretion and somatotroph proliferation, acting in balance with inhibitory somatostatin.
Somatostatin Hormone: An inhibitory peptide made by the hypothalamus, pancreatic δ cells and gut D cells that suppresses GH and TSH release, insulin and glucagon secretion, and most gastrointestinal secretion and motility.
Dopamine (prolactin-inhibiting hormone) Hormone: Dopamine released from tuberoinfundibular neurons into portal blood is the physiological prolactin-inhibiting hormone, holding lactotrophs under constant tonic inhibition.

Anterior pituitary hormones

Growth hormone (GH) Hormone: A 191-amino-acid protein from somatotrophs that promotes growth (largely by inducing IGF-1) and acts directly as a counter-regulatory, fat-mobilising, insulin-antagonising metabolic hormone.
Insulin-like growth factor 1 (IGF-1) Hormone: A 70-amino-acid peptide made mainly by the liver under GH stimulation that mediates the growth-promoting effects of GH and provides negative feedback on the GH axis.
Thyroid-stimulating hormone (TSH) Hormone: A glycoprotein from thyrotrophs that stimulates every step of thyroid hormone synthesis and release and the growth of the thyroid; serum TSH is the most sensitive single test of thyroid function.
Adrenocorticotropic hormone (ACTH) Hormone: A 39-amino-acid peptide cleaved from POMC in corticotrophs that stimulates cortisol and adrenal androgen secretion and maintains the size of the zona fasciculata and reticularis.
Follicle-stimulating hormone (FSH) Hormone: A glycoprotein gonadotropin that drives ovarian follicle growth and estradiol production in women and supports spermatogenesis via Sertoli cells in men.
Luteinizing hormone (LH) Hormone: A glycoprotein gonadotropin that stimulates testosterone production by Leydig cells and androgen production by theca cells; its mid-cycle surge triggers ovulation and formation of the corpus luteum.
Prolactin (PRL) Hormone: A protein hormone from lactotrophs that prepares the breast during pregnancy and drives milk synthesis after birth; it is the only pituitary hormone under predominant inhibitory (dopamine) control.

Posterior pituitary hormones

Antidiuretic hormone (ADH, vasopressin) Hormone: A nonapeptide made in the supraoptic and paraventricular nuclei and released from the posterior pituitary that conserves water by inserting aquaporin-2 into collecting ducts, and constricts arterioles at high concentrations.
Oxytocin Hormone: A nonapeptide from paraventricular (and supraoptic) neurons, released from the posterior pituitary, that contracts the uterus during labour and the myoepithelial cells of the breast during milk ejection — both via positive-feedback reflexes.

Thyroid & calcium-regulating hormones

Thyroxine (T4) Hormone: The main secretory product of the thyroid (≈90 % of output), a largely inactive prohormone with a week-long half-life that is converted in tissues to the active hormone T3.
Triiodothyronine (T3) Hormone: The biologically active thyroid hormone — 3–5 times more potent than T4 — produced mostly by peripheral deiodination of T4 and acting on nuclear receptors to raise metabolic rate and drive growth and development.
Calcitonin Hormone: A 32-amino-acid peptide from thyroid C cells that lowers plasma calcium by inhibiting osteoclasts; important in children and as a tumour marker, but of minor physiological importance in adults.
Parathyroid hormone (PTH) Hormone: An 84-amino-acid peptide from parathyroid chief cells that raises plasma calcium within minutes by acting on kidney and bone and by stimulating calcitriol synthesis, while lowering plasma phosphate.
Calcitriol (1,25-dihydroxyvitamin D₃) Hormone: The active steroid form of vitamin D, made by sequential hydroxylation in liver and kidney, that raises plasma calcium and phosphate chiefly by increasing their absorption from the gut.

Adrenal hormones

Cortisol Hormone: The principal glucocorticoid, secreted by the zona fasciculata under ACTH control in a circadian rhythm, that mobilises fuel during fasting and stress, maintains blood pressure, and restrains inflammation and immunity.
Aldosterone Hormone: The main mineralocorticoid, made by the zona glomerulosa in response to angiotensin II and high potassium, that promotes Na⁺ reabsorption and K⁺ and H⁺ secretion in the distal nephron, regulating extracellular volume, blood pressure and potassium.
Adrenal androgens (DHEA, DHEA-S, androstenedione) Hormone: Weak androgens from the zona reticularis — mainly DHEA and its sulfate — that are converted in peripheral tissues to testosterone and estrogens; they drive adrenarche and provide much of the androgen in women.
Epinephrine (adrenaline) Hormone: The main catecholamine of the adrenal medulla (≈80 % of its output), released within seconds of sympathetic activation to prepare the body for fight or flight by acting on α- and β-adrenergic receptors.
Norepinephrine (noradrenaline) Hormone: A catecholamine that is both the transmitter of most postganglionic sympathetic neurons and a minor (≈20 %) adrenal medullary hormone; acting mainly on α1, α2 and β1 receptors, it raises blood pressure by vasoconstriction.

Kidney & cardiac hormones

Renin Hormone: A proteolytic enzyme (not a classical hormone) secreted by the juxtaglomerular cells of the kidney when renal perfusion or sodium falls; it cleaves angiotensinogen to angiotensin I — the rate-limiting step of the renin–angiotensin–aldosterone system.
Angiotensin II Hormone: An octapeptide formed from angiotensin I by angiotensin-converting enzyme that raises blood pressure and conserves salt and water by vasoconstriction, aldosterone and ADH release, thirst and direct renal sodium retention.
Erythropoietin (EPO) Hormone: A glycoprotein produced mainly by renal peritubular interstitial fibroblasts in response to tissue hypoxia that stimulates red-cell production in the bone marrow.
Atrial natriuretic peptide (ANP) Hormone: A 28-amino-acid peptide released by atrial myocytes when they are stretched by increased blood volume; it lowers blood volume and pressure by promoting sodium and water excretion and vasodilation, opposing the renin–angiotensin–aldosterone system.

Pancreatic & incretin hormones

Insulin Hormone: The 51-amino-acid anabolic hormone of β cells — the only hormone that lowers blood glucose — which promotes uptake and storage of glucose, fat and amino acids and suppresses their release.
Glucagon Hormone: A 29-amino-acid peptide from islet α cells that raises blood glucose during fasting by stimulating hepatic glycogenolysis, gluconeogenesis and ketogenesis — the main counter-regulatory hormone to insulin.
Pancreatic polypeptide (PP) Hormone: A 36-amino-acid peptide from islet PP (F) cells, released after meals under vagal control, that inhibits exocrine pancreatic secretion and gallbladder contraction and contributes to satiety.
Glucagon-like peptide-1 (GLP-1) Hormone: An incretin hormone secreted by L cells of the distal small intestine and colon after meals that amplifies glucose-dependent insulin secretion, suppresses glucagon, slows gastric emptying and promotes satiety.

Gonadal & placental hormones

Testosterone Hormone: The principal male sex steroid, secreted by testicular Leydig cells under LH control, which drives male sexual differentiation, puberty, spermatogenesis, muscle and bone growth, and libido; many actions require conversion to DHT or estradiol.
Estrogens (estradiol, estrone, estriol) Hormone: C18 steroids — chiefly estradiol from ovarian granulosa cells — that drive female sexual development, proliferation of the endometrium, the mid-cycle LH surge and the maintenance of bone, and act in both sexes on bone, brain and liver.
Progesterone Hormone: A C21 steroid secreted by the corpus luteum after ovulation and by the placenta in pregnancy that converts the estrogen-primed endometrium to its secretory state and maintains pregnancy by keeping the uterus quiet.
Inhibin (and activin) Hormone: Glycoprotein hormones of the TGF-β family made by granulosa and Sertoli cells (and the placenta) that selectively suppress pituitary FSH secretion, allowing FSH and LH to be regulated independently.
Anti-Müllerian hormone (AMH) Hormone: A TGF-β-family glycoprotein from fetal Sertoli cells that causes regression of the Müllerian (paramesonephric) ducts in males; in women it is secreted by small growing follicles and serves as a marker of ovarian reserve.
Human chorionic gonadotropin (hCG) Hormone: A glycoprotein made by the syncytiotrophoblast from implantation onwards that acts like LH to rescue the corpus luteum, keeping progesterone high until the placenta takes over — and is the hormone detected by pregnancy tests.
Human placental lactogen (hPL) Hormone: A GH/prolactin-like protein from the syncytiotrophoblast, rising in proportion to placental mass, that makes the mother insulin-resistant so more glucose reaches the fetus and helps prepare the breasts for lactation.

Gastrointestinal & adipose hormones

Gastrin Hormone: A peptide from G cells of the gastric antrum that stimulates gastric acid secretion (mainly via histamine release from ECL cells) and growth of the gastric mucosa; acid in the antrum switches it off.
Secretin Hormone: A 27-amino-acid peptide from duodenal S cells, released when acid enters the duodenum, that stimulates bicarbonate-rich pancreatic and biliary secretion and inhibits gastric acid — "nature's antacid" and the first hormone ever discovered.
Cholecystokinin (CCK) Hormone: A peptide from I cells of the duodenum and jejunum, released by fat and protein digestion products, that contracts the gallbladder, relaxes the sphincter of Oddi, stimulates pancreatic enzyme secretion, slows gastric emptying and signals satiety.
Ghrelin Hormone: An acylated 28-amino-acid peptide secreted mainly by the gastric fundus during fasting that stimulates appetite and growth hormone release — the only known circulating hunger hormone.
Leptin Hormone: A 146-amino-acid protein secreted by adipocytes in proportion to fat stores that signals energy sufficiency to the hypothalamus, reducing appetite and permitting energy-expensive processes such as reproduction.

Thymic hormones

Thymosins and thymic hormones Hormone: A family of peptides (thymosin α1, thymulin, thymopoietin) produced by thymic epithelial cells that promote the differentiation and maturation of T lymphocytes.

Mnemonics

Anterior pituitary hormones: FLAT PiG: FSH, LH, ACTH, TSH, Prolactin, (i), GH. The B-FLAT hormones (FSH, LH, ACTH, TSH) come from Basophils; GH and prolactin come from acidophils.
Adrenal cortex zones: GFR from outside in — zona Glomerulosa, Fasciculata, Reticularis — make salt, sugar, sex (aldosterone, cortisol, androgens): "the deeper you go, the sweeter it gets".
Order of hormone loss in a compressive pituitary lesion: Go Look For The Adenoma Please: GH, LH, FSH, TSH, ACTH, Prolactin (prolactin is preserved or rises because the stalk effect removes dopamine inhibition).
Second messengers: cAMP (Gs): FLAT ChAMP — FSH, LH, ACTH, TSH, CRH, hCG, ADH (V2), MSH, PTH (+ calcitonin, GHRH, glucagon). IP₃ (Gq): GOAT HAG — GnRH, Oxytocin, ADH (V1), TRH, Histamine (H1), Angiotensin II, Gastrin. JAK–STAT: PIGGlET — Prolactin, Immunomodulators (cytokines), GH, G-CSF, Leptin, Erythropoietin, Thrombopoietin.
Hyperparathyroidism (hypercalcaemia): Stones, bones, abdominal groans, thrones and psychiatric overtones — renal calculi, bone pain/fractures, abdominal pain/constipation/pancreatitis, polyuria, depression/confusion.
Phaeochromocytoma: 5 Ps: Pressure (hypertension), Pain (headache), Perspiration, Palpitations, Pallor. Block α before β.
MEN syndromes: MEN1 = 3 Ps (Pituitary, Parathyroid, Pancreas). MEN2A = 2 Ps + 1 M (Parathyroid, Phaeochromocytoma, Medullary thyroid carcinoma). MEN2B = 1 P + 2 Ms (Phaeochromocytoma, Medullary carcinoma, Marfanoid habitus/Mucosal neuromas).
Functions of the hypothalamus: TAN HATS: Thirst and water balance, Adenohypophysis control, Neurohypophysis release, Hunger and satiety, Autonomic regulation, Temperature, Sexual behaviour (and circadian rhythm).

From the Corpus anatomy atlas — for education only.