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Growth hormone (GH) is secreted by the anterior pituitary under dual hypothalamic control: GHRH stimulates it and somatostatin restrains it. A third axis, ghrelin (from the stomach), enhances release through the growth hormone secretagogue receptor GHS-R.
A decisive physiological trait is pulsatility: GH is not released constantly but in pulses, predominantly at night. That pulsatile nature matters because physiological signaling and feedback mechanisms depend on the pattern, not just the total amount.
GH acts partly directly and partly through IGF-1 (insulin-like growth factor 1), produced mainly in the liver, which mediates many of its anabolic effects and serves as an integrating biomarker of the axis's activity.
A secretagogue is neither GH nor IGF-1: it is a molecule that stimulates the pituitary itself to modulate its natural release. This preserves —at least partially— pulsatility and physiological feedback, unlike administering the exogenous hormone directly.
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