Pituitary Hormones

Classification

Anterior: growth hormone(somatropin), octreotide(somatostatin analogue), GnRH agonists/antagonists, gonadotropins(FSH, LH, hCG), ACTH(corticotropin/tetracosactide). Posterior: vasopressin/desmopressin(ADH), oxytocin.

Growth Hormone Axis

Somatropin: recombinant GH — direct action + indirect via hepatic IGF-1(mediates most growth effects on bone/cartilage). Uses: GH deficiency + specific non-GH-deficient short stature(Turner syndrome, chronic renal insufficiency) — expanded beyond simple deficiency states. SE: glucose intolerance/insulin resistance(genuine counter-regulatory anti-insulin hormone, persists therapeutically), oedema, SCFE/scoliosis progression in growing children(mechanical stress on rapid bone growth), theoretical malignancy concern(IGF-1 signalling, unproven at standard dose).

Octreotide: suppresses pituitary GH release → acromegaly treatment(GH-secreting adenoma). SAME drug as GI antisecretory use(Constipation/Diarrhoea topic) — 2 distinct indications from somatostatin’s broadly inhibitory action.

GnRH Axis — THE ELEGANT PARADOX

Continuous/depot GnRH agonists(leuprolide, goserelin): paradoxically SUPPRESS LH/FSH despite being agonists — pituitary GnRH receptor needs PULSATILE stimulation(receptor desensitization/downregulation with sustained occupancy) → initial transient FLARE(↑LH/FSH/sex steroids, clinically important — counteracted with ANTIANDROGEN co-administered first weeks in prostate cancer to prevent tumour flare) → downregulation over 1-2wk → sustained SUPPRESSION. Agonist behaving like antagonist = function of PATTERN(pulsatile vs continuous), not drug’s intrinsic activity. Uses: prostate cancer(androgen deprivation), endometriosis, central precocious puberty, IVF(suppress premature LH surge) — pulsatile administration specifically can instead TREAT hypothalamic infertility(pattern determines effect).

GnRH antagonists(cetrorelix, ganirelix): IMMEDIATE direct block, no downregulation delay → immediate suppression WITHOUT flare — practical advantage in IVF where flare is counterproductive(higher cost).

ACTH

Used DIAGNOSTICALLY(ACTH stimulation test — distinguishes PRIMARY adrenal insufficiency [cortisol fails to rise, adrenal itself diseased] from SECONDARY/pituitary [adrenal intrinsically capable but chronically understimulated]), NOT routine replacement(direct cortisol/hydrocortisone simpler/more physiological for actual treatment).

Posterior Pituitary

Vasopressin(ADH)/desmopressin: V2 receptors(renal collecting duct) → aquaporin-2 insertion → water reabsorption(IDENTICAL mechanism as tolvaptan, here the AGONIST not antagonist). Central diabetes insipidus + nocturnal enuresis + mild haemophilia A/vWD(V2-mediated endothelial factor VIII/vWF release — Blood topic). Higher dose: V1 receptors(vascular smooth muscle) → vasoconstriction → separate use: vasopressor in septic shock + variceal bleeding(splanchnic vasoconstriction ↓portal pressure) — 2 SEPARATE uses via 2 SEPARATE receptor subtypes, same molecule.

Oxytocin: uterine smooth muscle oxytocin receptors → contraction. Labour induction/augmentation + postpartum haemorrhage control(uterine contraction compresses bleeding vessels). Structural similarity to vasopressin(both small related nonapeptides) → high-dose oxytocin cross-reacts at V2 → water intoxication/hyponatraemia risk with prolonged high-dose infusion(esp +large-volume IV fluids in labour) — structural-similarity off-target effect, SAME principle as elsewhere in pharmacology.

Clinical Relevance

GnRH agonist flare-then-suppress paradox = single most conceptually important fact: receptor response depends not just on WHICH drug occupies it, but the TEMPORAL PATTERN of occupancy — continuous vs pulsatile signalling producing OPPOSITE physiological outcomes — general pharmacological lesson, not narrowly GnRH-specific.

Classification (by Anterior/Posterior Origin)

Anterior pituitary hormones and analogues: growth hormone (somatropin), octreotide (somatostatin analogue, GH-suppressing), GnRH agonists/antagonists, gonadotropins (FSH, LH, hCG), ACTH (corticotropin/tetracosactide).

Posterior pituitary hormones: vasopressin/desmopressin (ADH), oxytocin.

Growth Hormone Axis

Growth hormone (somatropin): recombinant human GH, acting both directly on target tissues and indirectly via stimulating hepatic insulin-like growth factor-1 (IGF-1) production, which mediates most of GH’s growth-promoting effects on bone/cartilage. Used for GH deficiency (paediatric growth failure, adult GH deficiency) and specific non-GH-deficient short stature syndromes (Turner syndrome, chronic renal insufficiency) — a specific, examined point that GH use has expanded beyond simple deficiency states to certain conditions where supraphysiological GH still produces meaningful growth benefit despite normal endogenous GH secretion. Adverse effects: glucose intolerance/insulin resistance (GH is a genuine counter-regulatory, anti-insulin hormone at the physiological level, an effect that persists with therapeutic use), oedema, and slipped capital femoral epiphysis/scoliosis progression in growing children (mechanical stress on rapidly growing bone), plus a theoretical, actively-monitored concern about malignancy risk with IGF-1’s growth-promoting signalling, though this remains unproven at standard therapeutic dosing.

Octreotide (a somatostatin analogue, already introduced under Drugs for Constipation and Diarrhoea for its antisecretory GI use): suppresses GH release from the pituitary, the specific mechanism exploited in treating acromegaly (GH-secreting pituitary adenoma) — the same core drug, two genuinely distinct indications (GI secretory suppression versus pituitary hormone suppression), both flowing from somatostatin’s broadly inhibitory action across multiple endocrine/exocrine systems.

Gonadotropin-Releasing Hormone (GnRH) Axis

This axis illustrates one of the most elegant and frequently-examined pharmacological paradoxes in endocrinology: the same drug class, given differently, produces opposite net effects.

Continuous/pulsatile GnRH agonists (leuprolide, goserelin) given as a sustained-release depot: paradoxically suppress gonadotropin (LH/FSH) release, despite being agonists — the pituitary GnRH receptor requires pulsatile stimulation to sustain normal signalling (a receptor-desensitization/downregulation response to sustained, non-pulsatile occupancy), so continuous agonist exposure initially causes a transient surge in LH/FSH/sex steroids (the “flare” phenomenon, clinically important and specifically counteracted with an antiandrogen co-administered during the first weeks of prostate cancer treatment to prevent tumour flare) before receptor downregulation sets in over 1-2 weeks, after which sustained suppression of gonadotropins and downstream sex steroids results — the paradox of an agonist ultimately behaving like an antagonist, entirely a function of the pattern of receptor stimulation (pulsatile versus continuous) rather than any change in the drug’s intrinsic receptor activity. Used for prostate cancer (androgen deprivation), endometriosis, central precocious puberty, and controlled ovarian stimulation protocols (paradoxically, both to suppress premature LH surge during IVF cycles and, in pulsatile administration specifically, to treat hypothalamic causes of infertility — the pulsatile-versus-continuous distinction determining which effect is achieved).

GnRH antagonists (cetrorelix, ganirelix): immediately and directly block the GnRH receptor without requiring the downregulation delay — producing immediate gonadotropin suppression without the initial flare, a genuinely useful practical advantage over agonists in situations (like IVF protocols) where the flare phenomenon would be counterproductive, at generally higher direct cost.

Corticotropin (ACTH)

Used diagnostically (the ACTH stimulation test, assessing adrenal cortisol reserve — distinguishing primary adrenal insufficiency, where cortisol fails to rise because the adrenal itself is diseased, from secondary/pituitary insufficiency, where the adrenal gland is intrinsically capable of responding but chronically understimulated) rather than as routine replacement therapy, since direct cortisol/hydrocortisone replacement is simpler and more physiologically direct for actual adrenal insufficiency treatment.

Posterior Pituitary Hormones

Vasopressin (ADH)/desmopressin: acts on renal collecting duct V2 receptors, promoting aquaporin-2 insertion and water reabsorption (the identical V2-receptor mechanism already described in reverse for tolvaptan’s mechanism of action under Renal Pharmacology, here as the natural agonist rather than the antagonist) — used for central diabetes insipidus (inadequate endogenous ADH production, where exogenous replacement directly corrects the deficiency) and for nocturnal enuresis and specific bleeding disorders (mild haemophilia A/von Willebrand disease, via V2-receptor-mediated endothelial release of stored factor VIII/vWF, already covered under Blood). Vasopressin at higher doses also acts on V1 receptors on vascular smooth muscle, causing vasoconstriction — the basis for its separate use as a vasopressor in septic shock and for variceal bleeding (splanchnic vasoconstriction reducing portal pressure), a genuinely distinct receptor-mediated effect from its antidiuretic action, worth understanding as two separate pharmacological uses of the same molecule via two separate receptor subtypes.

Oxytocin: acts on uterine smooth muscle oxytocin receptors, stimulating contraction — used for labour induction/augmentation and for controlling postpartum haemorrhage (promoting uterine contraction to compress bleeding vessels at the placental site). Structural similarity to vasopressin (both are small, related nonapeptides) means high-dose oxytocin has some cross-reactivity at V2 receptors, producing a genuine, specifically-examined risk of water intoxication/hyponatraemia with prolonged high-dose oxytocin infusion (particularly in the context of large-volume IV fluid administration during labour) — a structural-similarity-driven off-target effect worth recognizing as the same “related peptide structure creates cross-reactivity” principle as elsewhere in pharmacology, rather than an isolated fact.

Clinical Relevance

The GnRH agonist flare-then-suppress paradox is the single most conceptually important fact in this topic — it demonstrates that a receptor’s response depends not just on which drug occupies it, but on the temporal pattern of occupancy, a principle worth carrying forward as a general pharmacological lesson (continuous versus pulsatile signalling producing opposite physiological outcomes) rather than a fact narrowly specific to GnRH analogues alone.

• GnRH receptor response over time: continuous agonist exposure causing initial flare then downregulation-driven suppression, vs antagonist’s immediate block.
• Vasopressin’s two receptor subtypes (V1 vasoconstriction vs V2 antidiuresis) mapped to its two separate clinical uses.

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