Serum levels of insulin-like growth factor (IGF) and IGF binding protein in constitutionally tall children and adolescents.

Serum insulin-like growth factor (IGF) and IGF binding protein (BP) levels were studied in 89 constitutionally tall children and adolescents (height greater than mean for age + 3 SD in 90% of the subjects). After separation by acidic gel filtration, the samples were assayed for IGF using a protein-binding assay (which measures mainly IGF I-related peptides) and for IGF BP by titration, in both cases using IGF I as tracer. The reference standard was a pool of normal adult serum with an assigned potency of 1 U IGF and 1 U IGF BP/ml. IGF levels increased with age in a manner similar to that in normal subjects, but at higher concentrations (P less than 0.02). The values were 0.77 +/- 0.05 (SE) U/ml in 1- to 5-yr-old children, 0.96 +/- 0.08 U/ml in 5- to 11-yr-old prepubescent children and 1.51 +/- 0.07 U/ml during puberty. BP levels developed in a different way from the above in that the increase with age was slight (0.70 +/- 0.07, 0.67 +/- 0.09, and 0.97 +/- 0.07 U/ml, respectively, for the three periods considered) and the levels were lower than those of normal subjects (P less than 0.005). After fusion of the epiphyses both IGF and BP levels were within the range of normal values. Thirteen girls underwent prolonged treatment (mean, 26 months) with ethinyl estradiol (250-300 micrograms/day). Deceleration in growth was accompanied by a progressive decrease in IGF levels throughout the period of therapy and a rise in BP levels during the first 6 months, after which they stabilized within the range of normal values. Although it is possible that excessive secretion of GH (which was not demonstrated by the usual tests) may be the cause of the elevated IGF levels during growth in these constitutionally tall subjects, it seems likely that the high IGF levels combined with the abnormally low levels of BP were responsible for their excessive growth.