Insulin-like growth factor I during growth in bulls.

In 10 bulls, changes in blood plasma concentrations of insulin-like growth factor I (IGF1) were studied during rearing and during the ensuing growth period. IGF1 continuously increased from 32 micrograms/l at the age of 15 d to 194 micrograms/l at the age of 307 d together with body weight. However, IGF1 was not related to daily rate of gain, which remained fairly constant during the growth period. An age-dependent increase was also observed for blood levels of insulin, thyroxine and triiodothyronine. The data suggest that insulin and thyroid hormones may be causally related to the age-dependent increase in IGF1 levels.