insulin-like growth factor 1 (IGF-1)
IGF-1 is the messenger that actually carries out much of growth hormone's work. If growth hormone is the manager giving orders, IGF-1 is the worker on the ground making bone and muscle cells multiply and enlarge.
It is a small protein produced mainly by the liver in response to growth hormone, though many tissues also make their own IGF-1 to act locally. Its name comes from its chemical resemblance to insulin: it can faintly mimic insulin's blood-glucose-lowering effects and binds a closely related receptor.
Because the liver releases IGF-1 steadily rather than in pulses, its blood level is a stable mirror of average growth hormone activity over the preceding day or so. This makes IGF-1 a far more practical lab test than growth hormone itself for screening growth disorders.
An important nuance: IGF-1 in the blood mostly travels bound to carrier proteins, especially IGFBP-3, which prolong its life and control how much is free to act. So total IGF-1 levels must be read against age, sex, and nutrition, since malnutrition lowers IGF-1 even when growth hormone is plentiful.
IGF-1 also exerts negative feedback: high IGF-1 tells the hypothalamus and pituitary to slow growth hormone release, closing the loop that keeps growth in balance.