Two hormones, one job
When a child grows, the visible change is height — but the chemistry behind it is a relay. The first runner is growth hormone (GH), made in the anterior pituitary and poured into the blood. GH does a little growing on its own, but most of its effect on the skeleton is indirect: it travels to the liver and tells it to release a second messenger, insulin-like growth factor 1 (IGF-1). IGF-1 is the molecule that actually reaches cartilage and muscle and tells them to divide and enlarge.
IGF-1 has an older name you'll still meet: somatomedin — literally the GH-dependent factor that mediates somatic (body) growth. Because IGF-1 lasts much longer in the blood than GH does, a single IGF-1 blood test is the steadiest way to gauge how much GH a person is making over a day.
The axis: brain on top, bone at the bottom
Above the pituitary sits the hypothalamus, which controls GH with two opposing signals. GHRH (growth-hormone-releasing hormone) is the accelerator — it tells the pituitary to fire. Somatostatin is the brake — it tells the pituitary to stay quiet. A third player, the stomach hormone ghrelin, also nudges GH release upward, which is one reason growth and appetite are linked.
Hypothalamus
GHRH (+) ───┐ ┌─── Somatostatin (−)
▼ ▼
Anterior pituitary
│ GH (bursts)
▼
Liver ──► IGF-1 ──► cartilage, muscle, bone
▲ │
└──── IGF-1 feeds back (−) ──┘
(IGF-1 also slows GHRH, boosts somatostatin)Notice the bottom of the diagram: IGF-1 doesn't just build the body, it also reports back. Rising IGF-1 dampens GHRH and stimulates somatostatin — a classic negative feedback loop that keeps growth from running away with itself.
Why GH comes in nightly bursts
GH is not released as a steady drip. It comes out in pulses, and the largest pulses happen during deep sleep, especially in the first hours of the night. Between pulses, blood GH can be almost undetectable. This is why a single random GH level tells you very little — you might have sampled a trough or a peak.
Because of this pulsing, doctors who suspect a GH problem rarely trust one measurement. They lean on IGF-1 (steady) or on provocative testing, where a stimulus is given to force a GH pulse and the response is watched. We'll meet that idea again when we look at deficiency and excess.