6.1 Hypothalamus & Pituitary Hormones

The Hypothalamic-Pituitary Axis

The hypothalamus is the bridge between the nervous and endocrine systems. It receives neural input from throughout the brain and translates that input into hormonal signals. The pituitary gland (hypophysis) sits in the sella turcica and is divided into two functionally distinct lobes: the adenohypophysis (anterior pituitary) and the neurohypophysis (posterior pituitary). The two lobes are connected to the hypothalamus by different mechanisms — a portal blood system for the anterior lobe and direct nerve tracts for the posterior lobe.

Hypophyseal Portal System

Hypothalamic releasing and inhibiting hormones are produced in discrete hypothalamic nuclei and travel through the hypophyseal portal venous system — a capillary network connecting the hypothalamus to the anterior pituitary. This portal arrangement delivers high concentrations of regulatory hormones directly to anterior pituitary cells before systemic dilution. The main hypothalamic hormones are:

A useful memory aid: CRH→ACTH, TRH→TSH, GnRH→FSH/LH, GHRH→GH, SST inhibits GH, dopamine inhibits PRL.

Anterior Pituitary Hormones

The anterior pituitary secretes six hormones whose effects can be grouped into two categories — trophic hormones that stimulate other endocrine glands, and direct-effect hormones:

1. Adrenocorticotropic Hormone (ACTH) — stimulates the adrenal cortex (zona fasciculata) to produce cortisol. Regulated by CRH and negative feedback from cortisol.
2. Thyroid-Stimulating Hormone (TSH) — stimulates the thyroid follicular cells to synthesize and release T3 and T4. Regulated by TRH and negative feedback from T3/T4.
3. Follicle-Stimulating Hormone (FSH) — stimulates ovarian follicle growth (females) and Sertoli cell spermatogenesis (males).
4. Luteinizing Hormone (LH) — triggers ovulation and corpus luteum formation (females); stimulates Leydig cell testosterone production (males).
5. Growth Hormone (GH, somatotropin) — acts directly on tissues and indirectly via hepatic Insulin-like Growth Factor-1 (IGF-1) to promote growth and anabolic metabolism.
6. Prolactin (PRL) — stimulates mammary gland development and milk synthesis. Uniquely under tonic inhibition by dopamine; removal of dopaminergic inhibition raises PRL.

FSH and LH together are the gonadotropins because they target the gonads.

Posterior Pituitary Hormones

The posterior pituitary is not a true gland — it is an extension of hypothalaptic neurons. Cell bodies in the supraoptic nucleus (primarily) and paraventricular nucleus synthesize Antidiuretic Hormone (ADH, vasopressin), while the paraventricular nucleus is the main source of oxytocin. These hormones travel down axons through the infundibulum as neurosecretory granules and are stored in axon terminals in the posterior pituitary, then released into systemic blood upon appropriate stimuli.

• ADH increases water permeability of the renal collecting ducts (via V2 receptors inserting aquaporin-2 channels), conserving water and concentrating urine. Major stimuli: increased plasma osmolality (detected by hypothalamic osmoreceptors) and decreased blood volume/pressure (via baroreceptors).
• Oxytocin stimulates uterine contraction during labor and milk ejection during lactation. It produces a positive-feedback loop (Ferguson reflex): cervical stretch releases more oxytocin, intensifying contractions until delivery.

Feedback Loops

Endocrine control relies on negative feedback:

• Long-loop feedback — the peripheral target gland hormone (e.g., cortisol, T4, testosterone) feeds back to suppress the hypothalamus and anterior pituitary.
• Short-loop feedback — the anterior pituitary trophic hormone (e.g., ACTH) feeds back to suppress the hypothalamus.
• Ultra-short-loop feedback — the hypothalamic hormone feeds back to suppress its own further release.

Loss of negative feedback explains common disorders: destruction of the thyroid gland raises TSH (primary hypothyroidism); an adrenal cortisol-secreting tumor suppresses CRH/ACTH (Cushing syndrome with low ACTH).

Clinical Pearls

• Sheehan syndrome — postpartum pituitary necrosis causes panhypopituitarism (hypothyroidism, hypoadrenalism, amenorrhea, failure to lactate).
• Prolactinoma — the most common functioning pituitary adenoma; dopamine agonists (cabergoline, bromocriptine) shrink the tumor by restoring inhibitory tone.
• Diabetes insipidus (central) — lack of ADH produces polyuria, polydipsia, and dilute urine; nephrogenic DI involves renal V2 receptor unresponsiveness.
• SIADH (Syndrome of Inappropriate ADH) — excess ADH causes water retention, hyponatremia, and concentrated urine.

Understanding the hypothalamic-pituitary axes is the scaffolding for every other endocrine topic on the PA-CAT — thyroid, adrenal, and reproductive physiology all map onto the same releasing-hormone → trophic-hormone → target-gland-hormone pattern.

Axis Integration: HPA, Growth, and Prolactin Control

The hypothalamic-pituitary-adrenal (HPA) axis is the canonical three-tier endocrine cascade and the template for every other axis on the PA-CAT. Hypothalamic CRH reaches the anterior pituitary through the hypophyseal portal system and stimulates corticotrophs to release ACTH, which drives the zona fasciculata to secrete cortisol. Cortisol then exerts long-loop negative feedback onto both the hypothalamus (suppressing CRH) and the anterior pituitary (suppressing ACTH/POMC). A defining feature is the diurnal rhythm: cortisol peaks early morning (6–8 AM) and nadirs near midnight. Loss of this rhythm — with elevated midnight cortisol — is an early sign of Cushing syndrome and is why a midnight salivary cortisol or dexamethasone suppression test is used diagnostically. Acute stress overrides feedback and diurnal patterning via noradrenergic input to the paraventricular nucleus, which is why critical illness, surgery, and severe trauma produce sustained hypercortisolism even without a tumor.

The growth hormone axis has a dual-control design unique among anterior pituitary hormones: GHRH stimulates somatotrophs to release GH, while somatostatin (SST) inhibits release. GH acts directly on tissues but most of its anabolic growth effect is mediated by IGF-1 produced in the liver via the GH receptor (JAK2/STAT5 pathway). IGF-1 then exerts long-loop negative feedback on both the hypothalamus (raising somatostatin) and the pituitary (lowering GH). The clinical discriminator PA-CAT items test is timing of excess: a GH-secreting pituitary adenoma before epiphyseal closure produces gigantism (excessive linear growth), while the same adenoma after closure produces acromegaly (enlarged hands, feet, jaw, frontal bossing). Diagnosis relies on failure to suppress GH with oral glucose and elevated IGF-1; somatostatin analogs (octreotide) are first-line medical therapy.

Prolactin is the one anterior pituitary hormone under tonic inhibitory control, and that control is dopamine acting through D2 receptors on lactotrophs. Any interruption of dopaminergic tone — dopamine antagonist drugs (antipsychotics, metoclopramide), hypothalamic stalk compression (craniopharyngioma), or pregnancy (high estrogen) — raises prolactin and can produce galactorrhea and amenorrhea. Notably, TRH also stimulates prolactin, which explains mild hyperprolactinemia in primary hypothyroidism: elevated TRH from loss of thyroid hormone feedback drives both TSH and prolactin upward, and the galactorrhea resolves with levothyroxine.

A common PA-CAT trap distinguishes the two pituitary lobes by mechanism, not just hormones. The adenohypophysis (anterior) is true glandular tissue that synthesizes its own hormones and is regulated by hypothalamic releasing/inhibiting hormones delivered through the hypophyseal portal venous system. The neurohypophysis (posterior) is modified neural tissue — it synthesizes nothing; instead, hypothalamic neurons in the supraoptic and paraventricular nuclei produce ADH and oxytocin, ship them down axons through the infundibulum, and store them in posterior terminals for later release. If a stem describes a lesion of the pituitary stalk, expect anterior hormone loss (portal delivery interrupted) with posterior sparing initially; a lesion of the posterior pituitary itself causes central diabetes insipidus (ADH storage lost) without affecting anterior hormones.