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Pituitary-Hypothalamic System - Pharmacology

Overview and Anatomy of the Axis

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Hypothalamic Hormones and Their Pituitary Effects

1. POMC-derived: ACTH, α-MSH
2. Somatotropic: GH, prolactin
3. Glycoprotein: TSH, FSH, LH

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I. ANTERIOR PITUITARY HORMONES

A. Adrenocorticotropic Hormone (ACTH / Corticotropin)

• Primarily a diagnostic tool to differentiate primary adrenal insufficiency (Addison disease - adrenal atrophy) from secondary adrenal insufficiency (inadequate pituitary ACTH)
• Cosyntropin (synthetic human ACTH) is the preferred preparation
• CRH is used diagnostically to distinguish Cushing syndrome from ectopic ACTH-producing tumors
• Also used for: infantile spasms, multiple sclerosis, resistant epilepsy

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B. Growth Hormone (GH / Somatotropin)

• Direct effects at target tissues
• Indirect effects via somatomedins - mainly IGF-1 (insulin-like growth factor 1), released from the liver
• In acromegaly (excess GH from pituitary tumor), IGF-1 levels are consistently elevated

• Route: subcutaneous or IM injection
• Bioavailability: ~70%; plasma t1/2 ~20 min, but biological t1/2 is longer; once-daily dosing is sufficient

• GH deficiency in children (pituitary dwarfism)
• Short stature from: Turner syndrome, Noonan syndrome, Prader-Willi syndrome, chronic renal insufficiency, small for gestational age, idiopathic short stature
• GH deficiency in adults (after surgery, trauma, or radiation)
• HIV-associated wasting/cachexia
• Short-bowel syndrome (malabsorption)

• Closed epiphyses in pediatric patients
• Active malignancy
• Proliferative or severe nonproliferative diabetic retinopathy
• Prader-Willi syndrome with severe obesity or respiratory impairment (risk of sudden death)
• Acute critical illness (post-surgery, trauma, respiratory failure)

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C. Somatostatin (Growth Hormone-Inhibiting Hormone)

• Acromegaly (excess GH) - first-line medical management
• Severe diarrhea/flushing in carcinoid tumors (and other neuroendocrine tumors)
• Bleeding esophageal varices (octreotide IV infusion)

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D. Gonadotropin-Releasing Hormone (GnRH) Analogs

• Leuprolide (leuprorelin), Goserelin, Buserelin, Histrelin
• Initial "flare" effect: temporary rise in LH, FSH, and sex steroids before suppression

• Prostate cancer (suppresses testosterone)
• Endometriosis (suppresses estrogen)
• Precocious puberty
• Prevention of premature LH surge in infertility protocols

• Cetrorelix, Ganirelix (injectable, used in infertility)
• Elagolix (oral) - for endometriosis; does not completely suppress pituitary function

• Women: hot flashes, sweating, diminished libido, depression, ovarian cysts; contraindicated in pregnancy
• Men: initial bone pain from testosterone surge; hot flashes, edema, gynecomastia, diminished libido

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E. Gonadotropins

• Follitropin alfa/beta (recombinant FSH) - stimulates ovarian follicular growth and maturation
• Urofollitropin (purified urinary FSH)
• Used in: infertility protocols (ovarian stimulation, controlled hyperstimulation)

• Menotropins (hMG) - urinary-derived mix of FSH + LH; used in infertility
• Chorionic gonadotropin (hCG) - acts like LH; triggers ovulation; also used in cryptorchidism and hypogonadism in males
• Lutropin alfa (recombinant LH)

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F. Prolactin

• Stimulated by TRH
• Tonically inhibited by dopamine (tuberoinfundibular DA pathway - hypothalamus to anterior pituitary)
• Neuroleptics/antipsychotics block D2 receptors → increased prolactin (hyperprolactinemia)

• Dopamine agonists are first-line: Bromocriptine and Cabergoline
• Cabergoline is preferred (longer half-life, better tolerated, more effective)
• Both also used for pituitary microadenomas
• Bromocriptine additionally indicated for type 2 diabetes (Cycloset formulation)

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G. TSH (Thyrotropin)

• Hypothyroidism: Levothyroxine (T4), liothyronine (T3)
• Hyperthyroidism:
  • Thioamides: Methimazole (preferred), PTU (first trimester of pregnancy); inhibit iodination and coupling of iodotyrosines
  • PTU also blocks peripheral T4 → T3 conversion
  • Iodide: "Wolff-Chaikoff effect" - inhibits thyroid hormone release; used in thyroid storm and pre-surgery
  • Radioactive iodine (131I): ablation; most patients develop hypothyroidism afterward
  • β-blockers (propranolol, metoprolol, atenolol): manage sympathetic symptoms in hyperthyroidism/thyroid storm

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II. POSTERIOR PITUITARY HORMONES

A. Oxytocin

• Stimulate uterine contractions and induce labor
• Manage postpartum hemorrhage

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B. Vasopressin (ADH - Antidiuretic Hormone)

• V2 receptor (kidney collecting tubules): ↑ water permeability (aquaporin-2 insertion) → water reabsorption
• V1 receptor (liver, vascular smooth muscle): vasoconstriction

• Diabetes insipidus (main use)
• Septic shock (vasopressor)
• Bleeding esophageal varices

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C. Desmopressin (DDAVP)

• Synthetic analog of vasopressin
• Minimal V1 activity → largely free of pressor effects
• Selective V2 agonist

• Diabetes insipidus (preferred over vasopressin)
• Nocturnal enuresis (bedwetting)
• Von Willebrand disease and mild hemophilia A (stimulates release of vWF and factor VIII)

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Summary Drug Table

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• Lippincott Illustrated Reviews: Pharmacology, Chapter 23 (pp. 766-788)
• Goodman & Gilman's The Pharmacological Basis of Therapeutics, Chapter 46

1. Goodman & Gilman's The Pharmacological Basis of Therapeutics
2. Katzung's Basic and Clinical Pharmacology, 16th Edition
3. Lippincott Illustrated Reviews: Pharmacology

• The answer I already gave you is sourced primarily from Lippincott Pharmacology and Goodman & Gilman - both are highly authoritative and cover the same topics as KD Tripathi, often in more detail.
• The core content on the pituitary-hypothalamic system (GnRH analogs, somatostatin analogs, dopamine agonists, posterior pituitary hormones, etc.) is the same across all standard pharmacology textbooks including KD Tripathi - the classification, mechanisms, and clinical uses do not differ significantly.
• If you want, I can present the same content in KD Tripathi's style - which is known for its concise classification tables, bulleted drug profiles, and exam-oriented summaries.

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PITUITARY-HYPOTHALAMIC SYSTEM PHARMACOLOGY

KD Tripathi Style

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OVERVIEW

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CLASSIFICATION OF DRUGS

A. Anterior Pituitary Hormones & Analogs

B. Posterior Pituitary Hormones & Analogs

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SECTION I: HYPOTHALAMIC REGULATORY HORMONES

High-Yield: All anterior pituitary peptide hormones are given parenterally (destroyed by GI proteases). Exception: oral GnRH antagonists (elagolix).

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SECTION II: DRUG PROFILES

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1. CORTICOTROPIN (ACTH)

• Diagnosis of adrenal insufficiency (primary vs. secondary)
• CRH: distinguish Cushing syndrome (pituitary) from ectopic ACTH
• Infantile spasms, MS, resistant epilepsy

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2. GROWTH HORMONE - SOMATROPIN

• Closed epiphyses
• Active malignancy
• Proliferative diabetic retinopathy
• Prader-Willi + severe obesity/respiratory impairment (sudden death risk)
• Acute critical illness

⚠️ High-Yield: In acromegaly, IGF-1 levels are consistently high (used to monitor, not just GH).

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3. SOMATOSTATIN ANALOGS

• Acromegaly (first-line medical therapy)
• Carcinoid tumors - severe diarrhea/flushing
• VIPoma, glucagonoma (other neuroendocrine tumors)
• Bleeding esophageal varices (octreotide IV)
• TSH-secreting pituitary adenoma

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4. GnRH AGONISTS

Paradox: Continuous use → receptor downregulation → chemical castration

• Women: hot flashes, sweating, ↓ libido, depression, ovarian cysts; contraindicated in pregnancy
• Men: initial testosterone flare → bone pain (give antiandrogen cover for 1st 2 weeks), hot flashes, gynecomastia, ↓ libido, osteoporosis

⚠️ Flare phenomenon: GnRH agonists initially raise LH/FSH/testosterone before suppression. Cover with flutamide/bicalutamide in prostate cancer for first 2 weeks.

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5. GnRH ANTAGONISTS

Advantage over agonists: Immediate suppression — no flare, no antiandrogen cover needed

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6. GONADOTROPINS

High-Yield: hCG mimics LH → triggers final oocyte maturation and ovulation in infertility protocols.

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7. PROLACTIN & DOPAMINE AGONISTS

TRH → ↑ Prolactin
Dopamine (tuberoinfundibular pathway) → ↓ Prolactin (tonic inhibition)
Antipsychotics (D2 blockers) → ↑ Prolactin (hyperprolactinemia)

• Hyperprolactinemia (both drugs)
• Prolactinoma (pituitary microadenoma/macroadenoma)
• Type 2 diabetes (bromocriptine)
• Parkinson disease (bromocriptine)
• Acromegaly (adjunct)

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SECTION III: POSTERIOR PITUITARY HORMONES

8. VASOPRESSIN (ADH) AND ANALOGS

• Synthesized in supraoptic (SON) + paraventricular (PVN) nuclei of hypothalamus
• Stored and released from posterior pituitary
• Released by: ↑ plasma osmolarity, ↓ blood volume, pain, stress

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9. VASOPRESSIN ANTAGONISTS (VAPTANS)

⚠️ Risk: Too-rapid correction of hyponatremia → osmotic demyelination syndrome (ODS). Correct Na⁺ by ≤10-12 mEq/L/day.

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10. OXYTOCIN

⚠️ Never give as IV bolus - causes severe hypotension. Always as slow IV infusion.

• Atosiban - competitive oxytocin receptor antagonist; used as tocolytic (prevent premature labor)

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QUICK REVISION: HIGH-YIELD EXAM POINTS

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