Physiology chapters
Hypothalamus, anterior and posterior pituitary
How the hypothalamus controls the anterior pituitary through releasing hormones and the posterior pituitary through nerve fibres, what each pituitary hormone does, and the classic disorders: acromegaly, gigantism, dwarfism, prolactinoma, hypopituitarism, diabetes insipidus and SIADH.
- The hypothalamus controls the anterior pituitary with releasing and inhibiting hormones carried in the hypophyseal portal veins; prolactin is the one held back mainly by dopamine.
- The anterior pituitary makes GH, prolactin, ACTH, TSH, LH and FSH; the posterior pituitary stores and releases ADH and oxytocin, made in the supraoptic and paraventricular nuclei.
- Key disorders: excess GH (gigantism/acromegaly), prolactinoma, Sheehan syndrome, diabetes insipidus (no ADH action) and SIADH (too much ADH).
On this page
Overview
The hypothalamus links the nervous and endocrine systems. It controls the pituitary gland, which in turn controls the thyroid, adrenal cortex, gonads, growth and lactation.
The pituitary has two parts with different origins and different control:
- Anterior pituitary (adenohypophysis): from Rathke's pouch (oral ectoderm). Controlled by hormones from the hypothalamus through portal blood vessels.
- Posterior pituitary (neurohypophysis): an extension of the brain. Hormones are made in the hypothalamus and travel down nerve axons to be stored and released here.
The hypothalamic-hypophyseal portal system
Hypothalamic neurons release hormones into capillaries in the median eminence. These drain into portal veins that carry them directly to the anterior pituitary, at high concentration.
| Hypothalamic hormone | Effect on the anterior pituitary |
|---|---|
| TRH | ↑ TSH (and prolactin) |
| CRH | ↑ ACTH |
| GnRH (pulsatile) | ↑ LH and FSH |
| GHRH | ↑ GH |
| Somatostatin (GHIH) | ↓ GH (and TSH) |
| Dopamine (PIH) | ↓ Prolactin |
Prolactin is the only anterior pituitary hormone mainly under inhibitory control, by dopamine. Cutting the pituitary stalk lowers all other anterior pituitary hormones but raises prolactin.
Anterior pituitary hormones
| Hormone | Main actions |
|---|---|
| Growth hormone (GH) | Growth of bone and tissues (through IGF-1); protein synthesis; fat breakdown; raises blood glucose |
| Prolactin | Milk production; inhibits GnRH (so suppresses ovulation) |
| ACTH | Cortisol (and adrenal androgen) secretion; growth of the adrenal cortex |
| TSH | Thyroid hormone synthesis and release; thyroid growth |
| LH | Ovulation and corpus luteum; testosterone from Leydig cells |
| FSH | Follicle growth; spermatogenesis (through Sertoli cells) |
Growth hormone in detail
- Regulation: GHRH increases and somatostatin decreases it. Release rises in deep sleep, fasting, exercise, hypoglycaemia, stress and high amino acids. It falls with high glucose, high free fatty acids and IGF-1 (negative feedback).
- Direct actions: raises blood glucose (reduces glucose uptake, diabetogenic), increases fat breakdown, increases protein synthesis.
- Indirect actions through IGF-1 (somatomedin C) from the liver: growth of cartilage and bone (at growth plates before they close) and other tissues.
GH is anti-insulin: it raises blood glucose. Its growth effects on bone are mainly through IGF-1.
Prolactin
Rises in pregnancy and with suckling. Drugs that block dopamine (some antipsychotics, metoclopramide) raise prolactin.
Posterior pituitary hormones
ADH (antidiuretic hormone, vasopressin)
- Made mainly in the supraoptic nucleus; stored in the posterior pituitary.
- Stimuli: rising plasma osmolarity (very sensitive: a 1–2% rise), sensed by hypothalamic osmoreceptors; falling blood volume or pressure (about 10% or more), sensed by baroreceptors; also pain, nausea, angiotensin II and some drugs.
- Inhibited by: low osmolarity, alcohol.
- Actions: V2 receptors in collecting ducts → aquaporin-2 → water reabsorption and concentrated urine; V1 receptors → vasoconstriction (at high levels).
Oxytocin
- Made mainly in the paraventricular nucleus.
- Milk ejection (let-down): suckling triggers a reflex release; contraction of myoepithelial cells around the alveoli of the breast.
- Uterine contraction in labour, by positive feedback as the cervix stretches (Ferguson reflex).
Prolactin makes milk; oxytocin ejects it.
Disorders
Growth hormone excess
- Before the epiphyses close: gigantism (very tall).
- After closure: acromegaly: enlarged hands, feet, jaw (prognathism) and nose, coarse face, large tongue, sweating, diabetes, high blood pressure, heart disease.
- Usually a pituitary adenoma. Diagnosed by a high IGF-1 and failure of GH to fall after a glucose load.
Growth hormone deficiency
- In children: pituitary dwarfism: short but normally proportioned, with normal intelligence (unlike hypothyroid cretinism).
- Laron dwarfism: GH receptor defect; GH high but IGF-1 low.
Prolactinoma
The commonest functioning pituitary tumour. In women: galactorrhoea and amenorrhoea, infertility. In men: low libido, erectile dysfunction. Treated with dopamine agonists (cabergoline, bromocriptine).
Pituitary tumour pressing on the optic chiasm
A large pituitary tumour grows upwards and presses on the crossing fibres of the optic chiasm, causing bitemporal hemianopia.
Hypopituitarism
How it happens: tumours, surgery, radiation, or Sheehan syndrome: pituitary infarction after severe postpartum haemorrhage (the gland is enlarged in pregnancy and vulnerable).
What you see: usually GH and gonadotrophins fail first, then TSH, then ACTH. In Sheehan syndrome, failure to lactate is often the first sign, then amenorrhoea, hypothyroidism and adrenal insufficiency.
Diabetes insipidus
| Type | Cause | Response to desmopressin |
|---|---|---|
| Central | No ADH (head injury, surgery, tumour, idiopathic) | Urine concentrates |
| Nephrogenic | Kidney does not respond (lithium, hypercalcaemia, inherited) | No response |
What you see: large volumes of dilute urine, thirst, high-normal or high plasma sodium if water intake is not kept up.
SIADH (syndrome of inappropriate ADH)
How it happens: too much ADH: small-cell lung cancer, lung infections, brain disorders, drugs (carbamazepine, SSRIs).
What you see: water retention with low plasma sodium (dilutional hyponatraemia), low plasma osmolarity and inappropriately concentrated urine, without oedema. Treated with fluid restriction.
Expect: the portal system, the releasing hormone for each pituitary hormone, dopamine and prolactin, GH regulation and its glucose effect, IGF-1, where ADH and oxytocin are made, the stimuli for ADH release, oxytocin vs prolactin, acromegaly vs gigantism, prolactinoma, Sheehan syndrome, central vs nephrogenic DI, and SIADH.
Quick revision
- Anterior pituitary: Rathke's pouch; portal control. Posterior: neural; stores ADH and oxytocin.
- Prolactin: inhibited by dopamine; rises after stalk section.
- GH: raised by deep sleep, fasting, exercise, hypoglycaemia; acts via IGF-1; raises glucose.
- ADH: supraoptic; osmolarity (1–2%) and volume (10%); aquaporin-2.
- Oxytocin: paraventricular; milk ejection and uterine contraction.
- Gigantism before, acromegaly after epiphyseal closure.
- Sheehan: postpartum pituitary infarction; failure to lactate.
- Central DI responds to desmopressin; nephrogenic does not. SIADH: low sodium.
Written by The House Job from standard physiology references (Guyton and Hall Textbook of Medical Physiology; BRS Physiology).
FAQ
Which pituitary hormone is under inhibitory control by dopamine?
Prolactin.
Where are ADH and oxytocin made?
In the hypothalamus: ADH mainly in the supraoptic nucleus and oxytocin mainly in the paraventricular nucleus. They are stored and released from the posterior pituitary.
What is the difference between gigantism and acromegaly?
Both are due to too much growth hormone; gigantism occurs before the growth plates close, acromegaly after.
What is Sheehan syndrome?
Infarction of the pituitary gland after severe bleeding at childbirth, causing hypopituitarism; failure to lactate is often the first sign.