SERUM INSULIN-LIKE GROWTH FACTOR-I IN 1030 HEALTHY-CHILDREN, ADOLESCENTS, AND ADULTS - RELATION TO AGE, SEX, STAGE OF PUBERTY, TESTICULAR SIZE, AND BODY-MASS INDEX

Serum levels of insulin-like growth factor-I (IGF-I) increase with age and pubertal development. The large variation in circulating IGF-I le vels in adolesence makes it difficult to use the IGF-I value of a sing le child in the assesment of his growth status. In addition, the inter ference of IGF-binding proteins in many IGF-I assays contributes to th is problem. We measured IGF-I in acid-ethanol-extracted serum from 103 0 healthy children, adolescents, and adults, employing a RIA that redu ces interference of IGF-binding proteins by using monoiodinated Tyr(31 )-[I-125]des-(1-3)IGF-I as radioligand. Mean serum IGF-I concentration s increased slowly in prepubertal children from 80-200 mu g/L with a f urther steep increase during puberty to approximately 500 mu g/L. Afte r puberty, a subsequent continuous fall in circulating IGF-I levels wa s apparent throughout adulthood to a mean of 100 mu g/L at the age of 80 yr (P < 0.0001). Girls had maximal IGF-I levels at 14.5 yr of age, whereas boys had peak IGF-I levels 1 yr later. This is almost 2 yr lat er than average peak height velocity. The large variation in serum IGF -I levels during puberty was diminished when data were separated accor ding to sex and Tanner stage of puberty. Interestingly, we found a sig nificant variation with age within the Tanner stages; there was an inc rease in serum IGF-I concentrations with age in the early pubertal sta ges and a decrease in the late stages (P < 0.05). Serum IGF-I increase d concomitantly with increasing testicular volume. Multiple regression analysis revealed that serum IGF-I levels predicted height velocity i n the following year (r = 0.33; P < 0.0001). Body mass index did not c orrelate significantly with serum IGF-I in prepubertal children in a m ultiple regression analysis. In conclusion, there was a significant va riation in serum IGF-I levels with age within a given Tanner stage of puberty in addition to the well known increase with increasing age or pubertal stage. Accordingly, the effects of sex, age, and puberty on s erum IGF-I cannot be separated into simple additive components when st udying 1030 children in a cross-sectional design. Thus, the age-, sex- , and puberty-corrected IGF-I values may, in fact, improve the use of serum IGF-I as a diagnostic tool to distinguish between a child with r etarded puberty and a GH-deficient individual.