Relationship of somatomedin-C concentration to bone age in boys with constitutional delay of growth

K. R. Rubin, J. M. Lichtenfels, S. K. Ratzan, M. Ozonoff, D. W. Rowe and D. E. Carey

Serum somatomedin-C (Sm-C) levels increase sharply during puberty, leading to difficulty in the interpretation of Sm-C values obtained from children who exhibit a discrepancy between chronological age (CA) and pubertal development. To evaluate the utility of assessing Sm-C levels on the basis of bone age (BA), we measured serum Sm-C levels in 44 boys with constitutional delay of growth (CDG). Levels of Sm-C were compared with the normative data of the Nichols Institute Reference Laboratories (NIRL), Los Angeles, by age category, substituting BA for CA. We found the mean Sm-C level in boys with CDG to be lower than that for NIRL normal subjects in each age category for both CA and BA, but the regression curve for Sm-C levels based on BA more closely approximated the NIRL regression curve than did the curve based on CA. The rise in Sm-C levels observed in NIRL normal subjects between CA 13 to 14 years is delayed in boys with CDG until CA 15 to 17 years only when a correction for BA is not made. We conclude that in boys with CDG, Sm-C levels should be interpreted on the basis of BA rather than CA, especially during the peripubertal period. The observation of blunted Sm-C levels in all age categories, even when BA was used, suggests that short children with presumed CDG may be at high risk for a "nonclassic" form of growth hormone deficiency.