Total and free iodothyronine levels of growing Thoroughbred foals: Effects of weaning and gender

The aim of this study was investigate the effect of growing associated with different gender on circulating total and free iodothyronine concentrations during the first 13 mo of age in foals. In addition, we investigated the evolution of circulating concentrations of thyroid hormones during the first 3 d of weaning. Blood was collected from 13 clinically healthy Thoroughbred foals every month. All foals were weaned at the 4 mo and blood samples were taken also at 24, 48 and 72 h after weaning. The results obtained showed growing effects for tri-iodothyronine (T3), thyroxine (T4), free tri-iodothyronine (fT3) and free thyroxine (fT4) values (P < .001).Serum T3 concentrations averaged respectively 2.89 and 0.29 nmol/L at 7 and 9 mo. Serum T4 concentrations averaged respectively 100.17 and 21.77 nmol/L at 1 and at 10 mo. Serum fT3 concentrations averaged respectively 6.96 and 1.50 pmol/L at 1 and 4 mo. Serum fT4 concentrations averaged respectively 31.40 and 4.93 pmol/L at 1 and 9 mo. Significant correlations between T3, T4, fT3 and fT4 with body weight (BW) and between T3, T4 and fT4 with age were observed.Weaning effects (P < .001) were shown for T3 and fT4 levels. No differences (P > .05) in T4 and fT3 levels were observed over the 3-day period. Gender effects (P < .001) were shown for T3, T4, fT3, and fT4 levels. Significant correlations between T4 and fT4 with BW and age were observed in colts and fillies. T3 concentrations were correlated with age only in colts and fT3 with BW only in colts. The results obtained seem to lend support to the recognized effects of growing and weaning in modulating the thyroid function of Thoroughbred foals. In fact, significant and differentiated effects of growing and weaning on total and free iodothyronine levels have been demonstrated.