Age and breed differences in thyroid hormones, insulin-like growth factor (IGF)-I and IGF binding proteins in female horses

A survey with horses was conducted to determine whether plasma concentrations of triiodothyronine (T-3), thyroxine (T-4), insulin-like growth factor (IGF)-I and IGF binding proteins (IGFBP) change as horses grow, mature sexually, and age. Jugular blood was sampled from Standardbred fillies and mares at ages 0, 1, 7, and 14 d, at 1, 2, 4, 5, 6, and 9 mo, and at 5 to 8 and 16 to 22 yr (n = 5 to 18). In a second survey, we measured the same variables in eight breeds of horses with markedly different adult body sizes, from Miniatures to Friesians. Plasma T-3, T-4, and IGF-I were determined by radioimmunoassays validated for horses, and IGFBP were estimated from radioligand assay following separation of the IGFBP by SDS-PAGE electrophoresis. Plasma T-3 decreased (P < .01) nearly continuously from 7.9 ng/mL on the day of birth to .9 ng/mL at 6 mo, and then changed little from .7 ng/mL at 9 mo to .5 ng/mL in mares 16 to 22 yr old. Similarly, T-4 declined (P < .01) from 233 ng/mL on the day of birth to 49 ng/mL at 14d and varied from 35 to 9 ng/mL among all of the older age groups. Plasma concentrations of IGF-I increased(P < .01) from 285 ng/mL on the day of birth to 572 ng/mL at 14 d, remained relatively constant until 9 mo of age (530 ng/mL), and then declined (P < .01) to low levels (295 ng/mL) in the oldest mares. We detected six IGFBP. The two smallest IGFBP (26 and 39-kDa) were highest during the first 14 d after birth and lowest (P < .01) in aged mares. The four larger IGFBP were lowest at birth and increased to the highest values during the most rapid growth period, but these changes were not significant (P > .20). In agreement with data for other species, our data suggest that IGF-I and IGFBP modulate growth in horses. Although there were impressive interbreed differences in circulating concentrations of T-3, T-4, IGF-I, and IGFBP, these were not related to differences in adult body size.