## Inhibition of Growth Hormone Secretion **Key Point:** Somatostatin (somatotropin-inhibiting factor, SIF) is the primary physiological inhibitor of GH secretion from somatotroph cells of the anterior pituitary. ### Mechanism of Somatostatin Action 1. Somatostatin is released by hypothalamic neurons into the hypophyseal portal blood 2. Binds to somatostatin receptors (SSTR2 and SSTR5) on somatotroph cells 3. Activates G~i~ proteins → ↓ cAMP → suppression of GH release 4. Also inhibits GHRH (growth hormone-releasing hormone) at the hypothalamic level ### Comparative Inhibitory Effects | Agent | Primary Effect | Strength | |-------|---|---| | **Somatostatin** | Direct pituitary inhibition | **Strongest** | | **IGF-1** | Negative feedback (moderate) | Moderate | | **Cortisol** | Suppresses GH at high levels | Weak to moderate | | **Thyroid hormone** | Permissive role; not primary inhibitor | Minimal direct inhibition | **High-Yield:** The GH axis is controlled by a dual hypothalamic regulation: GHRH (stimulatory) and somatostatin (inhibitory). Somatostatin is the dominant brake on GH secretion and is used clinically (octreotide, lanreotide) to suppress GH in acromegaly and other conditions. **Clinical Pearl:** Somatostatin also inhibits TSH, ACTH, and prolactin, making it a pan-inhibitor of anterior pituitary hormones except for FSH and LH. ### Why Other Options Are Weaker Inhibitors - **IGF-1:** Provides negative feedback but is less potent than somatostatin; acts at both hypothalamic and pituitary levels - **Cortisol:** At pathologically high levels (Cushing's syndrome) can suppress GH, but not a physiological inhibitor - **Thyroid hormone:** Required for normal GH action and secretion; not an inhibitor
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