Physiology of Anterior Pituitary Hormones¶

Chapter 390 | Part 12: Endocrinology and Metabolism · Part 12 – Endocrinology & Metabolism · Chapter 390

Key Clinical Points¶

The anterior pituitary produces six major hormones: PRL, GH, ACTH, LH, FSH, and TSH.
Pituitary hormones are secreted in a pulsatile manner; single random measurements (especially for GH) often fail to distinguish deficiency from normal range.
Prolactin (PRL) is unique as it is primarily under inhibitory hypothalamic control via dopamine.
Growth Hormone (GH) secretion peaks at night and declines by >80% during aging; IGF-1 is the preferred marker of GH action.
ACTH follows a circadian rhythm peaking at 6:00 a.m.; midnight cortisol is a key indicator for Cushing's syndrome.
Gonadotropin secretion requires pulsatile GnRH; continuous exposure leads to desensitization (basis for GnRH agonist use).
Estrogen has dual effects: chronic inhibition but midcycle stimulation of LH via positive feedback.
Pituitary development is governed by specific transcription factors: T-Pit (corticotropes), Prop-1/Pit-1 (somatotropes, lactotropes, thyrotropes), and SF-1/DAX-1 (gonadotropes).
Paracrine and autocrine controls (e.g., somatostatin, IGF-1) are critical but difficult to measure directly due to local concentrations.
The hypothalamic-pituitary portal system allows for the delivery of high-concentration hypothalamic peptides to the anterior pituitary without systemic dilution.

DEFINITION & OVERVIEW¶

• Master Gland: The anterior pituitary, along with the hypothalamus, orchestrates the regulation of multiple endocrine glands. • Six Major Hormones: (1) Proline (PRL), (2) Growth Hormone (GH), (3) Adrenocorticotropic Hormone (ACTH), (4) Luteinizing Hormone (LH), (5) Follicle-Stimulating Hormone (FSH), and (6) Thyroid-Stimulating Hormone (TSH). • Secretory Pattern: Hormones are released in a pulsatile manner, reflecting regulation by specific hypothalamic releasing factors. • Trophic Effects: Pituitary hormones elicit specific responses in peripheral tissues: adrenal, thyroid, gonads, liver, breast, and bone.

ANATOMY & DEVELOPMENT¶

• Anatomy: ◦ Located in the sella turcica; weighs ~600 mg. ◦ Proximity to cavernous sinuses, cranial nerves, and optic chiasm means expanding masses can cause significant central mass effects. • Vascularization: ◦ Anterior Pituitary: Supplied by the hypothalamic-pituitary portal plexus (short portal vessels) → allows high concentration of hypothalamic peptides without systemic dilution. ◦ Posterior Pituitary: Supplied by inferior hypophyseal arteries; directly innervated by hypothalamus via supraopticohypophyseal and tuberohypophyseal nerve tracts. • Developmental Transcription Factors: ◦ Prop-1: Induces Pit-1-specific lineages and gonadotropes. ◦ Pit-1: Determines cell-specific expression of GH, PRL, and TSH. ◦ T-Pit: Required for corticotrope development (POMC gene). ◦ SF-1 & DAX-1: Define gonadotrope cell development. ◦ TEF (Thyrotrope Embryonic Factor): Induces TSH expression.

HORMONAL RHYTHMS & CONTROL¶

• Circadian/Environmental Rhythms: ◦ All pituitary hormones are largely entrained to sleep and the 24-hour light-dark cycle. ◦ HPA Axis: ACTH and cortisol peak in early morning (6:00 a.m.) → nadir at midnight. ◦ Cushing's Syndrome Diagnosis: Characterized by elevated midnight cortisol levels compared to normal individuals. • Pulsatile Secretion: ◦ GnRH pulses are required for pituitary sensitivity; continuous exposure → desensitization of gonadotropes. ◦ Clinical Application: Long-acting GnRH agonists used in precocious puberty and prostate cancer to suppress gonadotropins. • Paracrine & Autocrine Control: ◦ Somatostatin (from pancreatic islet delta cells) inhibits insulin from nearby beta cells. /// Note: These are difficult to document due to local concentration issues. • Anatomic Influence: ◦ Portal vasculature ensures high-concentration exposure of pituitary to hypothalamic factors. ◦ Tissue organization (e.g., interdigitated Leydig cells) allows for localized hormone concentrations.

PROLACTIN (PRL)¶

• Synthesis & Source: ◦ Produced in lactotropes (~20% of anterior pituitary). ◦ Lactotrope/Somatotrope share a common precursor. • Regulation: ◦ Unique: Predominantly inhibited by hypothalamic dopamine (D2 receptors) → Pituitary stalk section leads to spontaneous hyperprolactinemia. ◦ TRH: Elicits PRL release within 15–30 min. • Physiological Effects: ◦ Lactation: Induces and maintains lactation; levels rise during pregnancy and fall after parturition. ◦ Reproductive Suppression: Inhibits GnRH, gonadotropins, and gonadal steroidogenesis → leads to hypoestrogenism, anovulation (women), and low testosterone/reduced sperm count (men). • Molecular Mechanism: ◦ Receptor is part of the type I cytokine receptor family. ◦ Signaling: Binding → dimerization → JAK → STAT translocation → gene activation. ◦ Mutations: Cause PRL insensitivity, hyperprolactinemia, and oligomenorrhea.

GROWTH HORMONE (GH)¶

• Synthesis & Source: ◦ Produced in somatotropes (up to 50% of anterior pituitary). ◦ Pit-1 dependent. • Regulation: ◦ GHRH: Stimulates synthesis/release; GPCR signaling via cAMP. ◦ Ghrelin: Stimulates GH release directly and via GHRH induction. ◦ Somatostatin (SRIF): Inhibits GH (SST2, SST5) and TSH. ◦ Feedback: IGF-1 inhibits GH; Estrogen stimulates GH; Chronic glucocorticoids suppress GH. • Physiology: ◦ Metabolism: Increases protein synthesis/nitrogen retention; impairs glucose tolerance (antagonizes insulin); stimulates lipolysis. ◦ Growth: Stimulates epiphyseal prechondrocyte differentiation → local IGF-1 production. • Clinical Measurement Pitfalls: ◦ Pulsatile nature: 50% of daytime samples in healthy subjects are undetectable. ◦ Age/Obesity: Often undetectable (<1 μg/L) in elderly or obese patients. ◦ Diagnosis: Single random GH measurements cannot distinguish adult GH deficiency from normal range; use IGF-1 as a stable marker.

ACTH & TSH¶

• ACTH: ◦ Derived from POMC gene (266 amino acids). ◦ Stimulated by CRH, AVP, cytokines. ◦ Inhibited by Glucocorticoids. • TSH: ◦ Glycoprotein with α and β subunits (211 amino acids). ◦ Stimulated by TRH. ◦ Inhibited by T, thyroid hormones, dopamine, and somatostatin.

GONADOTROPINS: FSH AND LH¶

• Synthesis: ◦ Produced in gonadotropes (SF-1, DAX-1 dependent). ◦ LH (204 amino acids) and FSH (210 amino acids). • Regulation: ◦ Require pulsatile GnRH. ◦ Continuous exposure → desensitization. • Estrogen Dynamics: ◦ Chronic: Inhibitory. ◦ Midcycle rise: Positive feedback → LH surge.

DIAGNOSTIC APPROACH¶

Assessment of Pulsatile Hormones (e.g., GH):
Avoid relying on single random measurements.
Use IGF-1 as the primary marker for GH action.
Assessment of ACTH/Cortisol:
Measure cortisol at midnight to identify Cushing's syndrome → elevated levels indicate pathology.
Assessment of TSH:
Must be interpreted in conjunction with T4 and T3 levels.
Low T4/T3 + High TSH = Primary Hypothyroidism.
Low T4/T3 + Low/Normal TSH = Secondary Hypothyroidism (pituitary defect).
Assessment of Prolactin:
Use to evaluate pituitary stalk integrity; hyperprolactinemia suggests stalk compression or mass effect.

MANAGEMENT & TREATMENT¶

Treatment of Precocious Puberty/Prostate Cancer:
Use long-acting GnRH agonists → causes desensitization of gonadotropes.
Glucocorticoid Replacement:
Administer larger doses in the morning than in the afternoon to mimic natural diurnal production.
Monitoring Growth:
Utilize IGF-1 levels to monitor GH action, as it is less affected by fluctuations and more reflective of long-term growth status.

KEY PEARLS & HIGH-YIELD POINTS¶

• Dopamine's Role: It is the primary inhibitor of Prolactin; loss of dopamine tone (e.g., stalk transection) → hyperprolactinemia. • GnRH Pulse Frequency: Determines the specific ratio of LH vs. FSH secretion. • Somatostatin Subtypes: SST2 and SST5 preferentially suppress GH and TSH, while SST5 also suppresses ACTH. • Glucose Suppression Test: Used to assess GH; a glucose load should suppress GH to <0.7 μg/L in women and <0.07 μg/L in men.

Reference Tables¶

TABLE 390-1 Anterior Pituitary Hormone Expression and Regulation CELL Tissue-specific transcription factor…¶

Harrison's 22e, p.2988

CELL	CORTICOTROPE	SOMATOTROPE	LACTOTROPE	THYROTROPE	GONADOTROPE
Tissue-specific transcription factor	T-Pit	Prop-1, Pit-1	Prop-1, Pit-1	Prop-1, Pit-1, TEF	SF-1, DAX-1
	6 weeks	8 weeks	12 weeks	12 weeks
Hormone	POMC	GH	PRL	TSH	FSH, LH
	Polypeptide	Polypeptide	Polypeptide	Glycoprotein α, β subunits
Amino acids	266 (ACTH 1–39)	191	198	211	210, 204
	CRH, AVP, cytokines	GHRH, ghrelin	Estrogen, TRH, VIP	TRH
Inhibitors	Glucocorticoids	Somatostatin, IGF-1	Dopamine	T, T, dopamine, 3 4 somatostatin, glucocorticoids	Sex steroids, inhibin
	Adrenal	Liver, bone, other tissues	Breast, other tissues	Thyroid
Trophic effect	Steroid production	IGF-1 production, growth induction, insulin antagonism	Milk production	T synthesis and secretion 4	Sex steroid production, follicle growth, germ cell maturation
	ACTH, 4–22 pg/L	<0.5 μg/La	M <15 μg/L; F <20 μg/L	0.1–5 mU/L