Chapter 2 of 5 - Endocrine System Course
The hypothalamus and pituitary gland form the central command unit of the endocrine system - translating neural signals into hormonal responses that regulate growth, metabolism, reproduction, and stress.
The hypothalamus is a small region at the base of the diencephalon, sitting just below the thalamus and above the pituitary gland. Despite weighing only about 4 grams, it serves as the primary link between the nervous system and the endocrine system. It receives input from virtually every part of the brain - including the limbic system, cerebral cortex, and brainstem - and converts these neural signals into hormonal outputs.
The hypothalamus controls the pituitary gland through two distinct mechanisms:
Beyond endocrine control, the hypothalamus also regulates body temperature, hunger and satiety, thirst, circadian rhythms, and autonomic nervous system function - making it one of the most functionally diverse structures in the brain.
The hypothalamus produces several small peptide hormones (and one amine) that travel through the hypophyseal portal system to reach the anterior pituitary. These hormones either stimulate (releasing) or suppress (inhibiting) secretion of specific anterior pituitary hormones.
| Hypothalamic Hormone | Abbreviation | Action on Anterior Pituitary |
|---|---|---|
| Gonadotropin-releasing hormone | GnRH | Stimulates release of FSH and LH |
| Thyrotropin-releasing hormone | TRH | Stimulates release of TSH (and prolactin) |
| Corticotropin-releasing hormone | CRH | Stimulates release of ACTH |
| Growth hormone-releasing hormone | GHRH | Stimulates release of GH |
| Somatostatin (GHIH) | SS / GHIH | Inhibits release of GH (and TSH) |
| Dopamine (PIH) | DA / PIH | Inhibits release of prolactin |
A key clinical point: dopamine is the primary inhibitor of prolactin. If the pituitary stalk is severed (cutting off dopamine delivery), prolactin levels rise - unlike every other anterior pituitary hormone, which would fall without hypothalamic input. This is why prolactin is the only anterior pituitary hormone under predominantly inhibitory control.
Quick Check
Which hypothalamic hormone primarily inhibits prolactin secretion from the anterior pituitary?
The anterior pituitary originates from Rathke's pouch - an ectodermal outgrowth of the oral cavity (pharyngeal epithelium). It is a true glandular structure that synthesizes, stores, and secretes its own hormones. It accounts for about 80% of the pituitary gland by weight and produces six major hormones.
A helpful mnemonic for the anterior pituitary hormones: "FLAT PiG" - FSH, LH, ACTH, TSH, Prolactin, GH.
| Hormone | Cell Type | Target | Primary Action |
|---|---|---|---|
| Growth Hormone (GH) | Somatotrophs | Liver, bone, muscle | Promotes growth via IGF-1; increases blood glucose |
| TSH | Thyrotrophs | Thyroid gland | Stimulates thyroid hormone synthesis (T3/T4) |
| ACTH | Corticotrophs | Adrenal cortex | Stimulates cortisol secretion (zona fasciculata) |
| FSH | Gonadotrophs | Ovaries / testes | Follicle development (F) / spermatogenesis (M) |
| LH | Gonadotrophs | Ovaries / testes | Ovulation and progesterone (F) / testosterone (M) |
| Prolactin | Lactotrophs | Mammary glands | Stimulates milk production; inhibits GnRH |
Note that somatotrophs make up roughly 50% of the cells in the anterior pituitary - making GH the most abundantly produced anterior pituitary hormone. This is clinically relevant because pituitary adenomas most commonly arise from somatotrophs (GH-secreting) or lactotrophs (prolactin-secreting).
Fill in the Blank
The anterior pituitary develops from________- an ectodermal outgrowth of the oral cavity, while the posterior pituitary develops from neural ectoderm (a downward extension of the brain).
The posterior pituitary is not a true gland - it is a downward extension of neural tissue from the hypothalamus. It does not synthesize any hormones. Instead, it stores and releases two hormones that are produced by neurosecretory cells in the hypothalamus:
Hypothalamus
Supraoptic nucleus produces ADH - Paraventricular nucleus produces oxytocin
Hypothalamo-hypophyseal tract
Axons transport hormones down to posterior pituitary
Posterior Pituitary (storage and release)
Hormones stored in axon terminals - released into blood on neural signal
| Hormone | Source Nucleus | Target | Primary Action |
|---|---|---|---|
| ADH (vasopressin) | Supraoptic nucleus | Kidneys (collecting ducts) | Increases water reabsorption - concentrates urine |
| Oxytocin | Paraventricular nucleus | Uterus, mammary glands | Uterine contractions (labor) and milk letdown reflex |
ADH acts on V2 receptors in the collecting ducts of the kidney to insert aquaporin-2 channels, allowing water to be reabsorbed from the filtrate. At higher concentrations, ADH also acts on V1 receptors in vascular smooth muscle to cause vasoconstriction - hence the alternative name "vasopressin."
Oxytocin operates through a positive feedback mechanism during childbirth - uterine contractions push the baby against the cervix, which triggers more oxytocin release, which triggers stronger contractions, and so on until delivery.
Quick Check
A patient has lost the ability to concentrate urine and is producing large volumes of very dilute urine. Which hormone deficiency is most likely responsible?
The hypothalamus and pituitary communicate with peripheral endocrine glands through tightly regulated negative feedback loops. When the end-organ hormone rises to a sufficient level, it feeds back to suppress both the hypothalamus and the pituitary, reducing further stimulation. Three major axes govern this communication:
HPA Axis
Hypothalamus
CRH
Anterior Pituitary
ACTH
Adrenal Cortex
Cortisol
Cortisol feeds back (-)
HPT Axis
Hypothalamus
TRH
Anterior Pituitary
TSH
Thyroid Gland
T3 / T4
T3/T4 feeds back (-)
HPG Axis
Hypothalamus
GnRH
Anterior Pituitary
FSH / LH
Gonads
Estrogen / Testosterone
Sex steroids feed back (-)
Understanding these axes is critical for interpreting lab values. For example, a patient with high cortisol and low ACTH likely has a primary adrenal tumor (the cortisol is suppressing ACTH via negative feedback). Conversely, high cortisol and high ACTH suggests an ACTH-secreting pituitary adenoma (Cushing disease), where the tumor escapes normal feedback control.
Fill in the Blank
In the HPA axis, the hypothalamus secretes CRH, which stimulates the anterior pituitary to release ACTH, which in turn stimulates the adrenal cortex to produce________- a glucocorticoid that then feeds back to suppress both CRH and ACTH.
Disorders of the hypothalamic-pituitary system often result from either excess or deficiency of specific hormones. Understanding the normal physiology makes these conditions straightforward to predict.
Excess growth hormone in children (before epiphyseal plate closure) causes gigantism - proportional overgrowth of the entire skeleton. In adults (after plate closure), it causes acromegaly - enlargement of the hands, feet, jaw (prognathism), and soft tissues. Most cases result from a GH-secreting pituitary adenoma. Diagnosis involves measuring serum IGF-1 and performing an oral glucose tolerance test (GH should normally be suppressed by glucose but is not in acromegaly).
Deficiency of growth hormone in childhood leads to proportional short stature - the child has normal body proportions but is significantly shorter than expected. Unlike achondroplasia (a genetic skeletal dysplasia causing disproportionate dwarfism), pituitary dwarfism can be treated with exogenous recombinant GH (somatropin) if diagnosed early.
Excessive ADH secretion - often caused by small cell lung cancer, CNS disorders, or certain drugs - leads to excessive water retention, dilutional hyponatremia (low serum sodium), concentrated urine, and decreased serum osmolality. Treatment involves fluid restriction and, in severe cases, vasopressin receptor antagonists (vaptans) or hypertonic saline.
Central diabetes insipidus results from inadequate ADH production (hypothalamic/pituitary damage), while nephrogenic diabetes insipidus results from kidney resistance to ADH (often from lithium use or genetic mutations in V2 receptors). Both produce massive volumes of dilute urine (up to 20 L/day), severe thirst, and hypernatremia. Central DI responds to desmopressin (synthetic ADH) - nephrogenic DI does not.
During pregnancy, the pituitary gland enlarges significantly (primarily due to lactotroph hyperplasia) but its blood supply does not increase proportionally. If the mother experiences severe postpartum hemorrhage and hypotension, the enlarged pituitary can undergo ischemic necrosis. The result is panhypopituitarism - deficiency of all anterior pituitary hormones. The earliest sign is often failure to lactate (loss of prolactin), followed by fatigue, amenorrhea, and signs of hypothyroidism and adrenal insufficiency.
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