IGF-1 Negative Feedback

IGF-1 negative feedback is the closing arm of the somatotropic axis. Circulating IGF-1, most of it hepatic, acts back at two levels: on the somatotroph, where it suppresses growth hormone gene transcription and secretion, and on the hypothalamus, where it raises somatostatin tone and reduces GHRH drive. Growth hormone also feeds back on itself over a shorter loop, so a large pulse tends to be followed by a refractory interval rather than another pulse.

The experiments of nature are unusually clean here. In Laron syndrome the growth hormone receptor cannot signal, IGF-1 is very low, feedback is absent, and growth hormone concentrations run high despite complete resistance. The reverse is seen with sustained exogenous growth hormone or with mecasermin, where a high IGF-1 shuts endogenous secretion down. In acromegaly the adenoma escapes the loop altogether, which is exactly why an oral glucose load fails to suppress it.

Feedback is the reason stimulating the axis and replacing the hormone are not the same intervention. A secretagogue raises growth hormone inside a system that is still counting, so IGF-1 accumulation eventually damps the response and imposes a ceiling. Injected growth hormone is outside the loop; it suppresses the endogenous contribution instead of adding to it, and the resulting exposure is continuous rather than pulsatile.

The error worth naming is the assumption that intact feedback makes an intervention self-limiting and therefore safe. Feedback constrains the height of a pulse, not the integrated exposure over months, and the adverse effects that matter in this axis follow the integral. It is also frequently forgotten that feedback runs in the other direction after withdrawal: the axis suppressed by exogenous hormone does not necessarily resume its previous output the moment dosing stops.