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

This study examines the variances in plasma levels of thyroid hormones, insulin-like growth factor (IGF)-I and IGF binding proteins in female horses as they age and mature. The research discovered significant variations, which however, did not directly correlate with adult body size.

Methodology

• The study was conducted through two surveys. One involving Standardbred fillies and mares at different ages from birth to 22 years. The second survey involved horses from eight different breeds. Samples were collected from horses of markedly different adult body sizes, from Miniatures to Friesians.
• Jugular blood was sampled and the plasma concentrations of triiodothyronine (T3), thyroxine (T4), insulin-like growth factor (IGF)-I and IGF binding proteins (IGFBP) were evaluated.
• Identification and measurement of plasma T3, T4, and IGF-I was carried out using radioimmunoassays validated for horses. IGFBP were evaluated from radioligand assay after separating the IGFBP by SDS-PAGE electrophoresis.

Findings

• Plasma T3 decreased consistently from 7.9 ng/mL on the day of birth to 0.5 ng/mL in mares 16 to 22 years old.
• Plasma T4 similarly fell from 233 ng/mL on the day of birth to 49 ng/mL at 14 days. It varied from 35 to 9 ng/mL in all adult age groups.
• Plasma concentrations of IGF-I rose from 285 ng/mL on the day of birth to 572 ng/mL at 14 days, maintained this level until 9 months of age (530 ng/mL), and then declined (P < .01) to low levels (295 ng/mL) in the oldest mares.
• Six IGFBP were detected. The two smallest IGFBP (26 and 39 kDa) were highest during the first 14 days after birth and lowest in aged mares. The four larger IGFBP had their lowest values at birth and increased to the highest levels during the most rapid growth phase, though these changes were not significant.

Conclusion

• Numerous changes were observed in plasma T3, T4, IGF-I, and IGFBP concentrations as the horses grew, matured sexually, and aged.
• While there were noticeable differences between breeds in circulating concentrations of these hormones and proteins, these variances did not necessarily represent their differences in adult body size.
• This study provides evidence that IGF-I and IGFBP are important factors modulating growth in horses, consistent with data from other species.