Growth Hormone Physiology Across the Lifespan

Growth hormone (GH) is released by somatotroph cells in the anterior pituitary. Growth-hormone-releasing hormone (GHRH) stimulates its release, while somatostatin suppresses it. GH acts directly on several tissues, but much of its growth-promoting effect occurs through insulin-like growth factor-1 (IGF-1). The liver is a major source of circulating IGF-1 after GH stimulation. GH is the signal sent from the pituitary; IGF-1 carries much of that signal to bone, cartilage, muscle, and other peripheral tissues. This distinction explains why IGF-1 is useful in assessment. GH secretion fluctuates substantially, so a single random GH level may not represent the patient's overall GH activity. IGF-1 provides a more stable screening measure, although it cannot establish GH deficiency by itself.