Somatomedin-C/IGF-I measured by radioimmunoassay and somatomedin bioactivity in adolescents with insulin dependent diabetes compared with puberty matched controls.

Plasma concentrations of insulin like growth factor-I in the adolescent diabetic remain controversial. IGF-I levels were determined by radioimmunoassay in 81 insulin dependent adolescent diabetics (49 boys and 32 girls) and compared with 75 puberty stage matched normal controls. Plasma somatomedin bioactivity was determined by cartilage bioassay in a smaller group of 10 normal and 25 diabetic subjects. IGF-I concentrations increased in the normals during puberty and there were no observed differences between the sexes. (P1, 0.77 +/- 0.08; P2, 1.33 +/- 0.2; P3, 1.59 +/- 0.16; P4, 1.76 +/- 0.16; P5, 2.24 +/- 0.09). IGF-I also increased in the diabetics reaching a peak level at stage three in both girls and boys. Combining the data from both sexes the diabetics had significantly lower levels of IGF-I compared to controls at stages; (P1, 0.67 +/- 0.05, ns. P2, 0.92 +/- 0.1, p less than 0.05; P3, 1.16 +/- 0.09, p less than 0.01; P4, 0.73 +/- 0.11, p less than 0.001; P5, 1.13 +/- 0.11, p less than 0.001). Plasma somatomedin bioactivity was elevated in the normals but inhibitory responses were observed in all but two of the diabetic subjects, highlighting the presence of inhibitory factors in diabetic plasma. HbA1C levels rose in the diabetics during puberty, however using covariance analysis there was no relationship between IGF-I and HbA1C when changes relating to puberty were excluded. The importance of controlling for pubertal stage as opposed to age is noted when assessing IGF-I status in the diabetic adolescent.