Plasma Insulin-Like Growth Factor-I/Somatomedin-C in Acromegaly: Correlation With the Degree of Growth Hormone Hypersecretion*

Abstract

We measured plasma insulin-like growth factor I/somatomedin-C (IGF-I/SmC) concentrations and mean 24-h GH secretion serially before and during therapy with the long-acting somatostatin analog SMS 201-995 in 21 patients with acromegaly. When mean plasma GH was elevated above 12.0 +/- 0.6 (+/- SE) micrograms/L, plasma IGF-I/SmC concentrations were uniformly high, but a decline of mean plasma GH below this value was accompanied by a linear decrease in IGF-I/SmC concentrations (r = 0.89; P less than 0.001). Even mildly abnormal mean GH concentrations (greater than 4.6 but less than 10 micrograms/L) were accompanied by high plasma IGF-I/SmC values. The log dose-response interrelation between mean 24-h plasma GH and IGF-I/SmC concentrations was linear (r = 0.86; P less than 0.001). We conclude that 1) an excellent log dose-response correlation between mean 24-h plasma GH and IGF-I/SmC concentrations is present in patients with acromegaly; 2) normalization of plasma IGF-I/SmC occurs only in patients with mean daily GH output within the normal range; and 3) determination of plasma IGF-I/SmC is an accurate indicator of normalcy of GH secretion and should be used in the diagnosis of active acromegaly as well as in monitoring the progress of therapy.