Insulin-like growth factor measurements in the evaluation of growth hormone secretion.

The measurement of circulating levels of the insulin-like growth factors (IGF-I and IGF-II) in the evaluation of GH secretion is discussed. GH is an important regulator of circulating IGF-I levels in humans. Circulating IGF-I levels are low in patients with GH deficiency, elevated in those with GH excess, and correlated well with physiologic changes in GH secretion. The levels also increase in response to administration of exogenous GH or GH-releasing hormone. However, IGF-I levels are age dependent and subject to regulation by other hormones and nutritional variables; these features complicate the interpretation of IGF-I levels in individual patients and limit the usefulness of these measurements, particularly for determining GH deficiency in young children. Circulating IGF-II levels are not GH dependent and, therefore, their measurement is of little clinical utility in assessing GH secretion.