Plasma total T3 and T4 concentrations in goats at different physiological stages, as affected by the energy intake

The effects of different concentrate levels in the diet on plasma 3-3'-5-triiodothyronine (T3) and thyroxine (T4) concentrations in does of different physiological conditions were investigated. Blood samples were taken starting in autumn from 20 goats fed hay ad libitum, during the dry period (30 days and 3 days prior to the synchronized oestrus; D1 and D2, respectively), pregnancy (week 13, week 17 and week 21 of pregnancy; P1, P2 and P3, respectively) and lactation (5 weeks and 13 weeks following parturition; L1 and L2, respectively). The control group (C, n = 10) was fed either 0.2 (in periods D1, D2 and P1) or 0.4 kg maize grain(head per day) (in P2, P3, L1 and L2). The high energy diet group (H, n = 10) was fed either 0.4 (in D1, D2 and P1) or 0.7 kg maize grain(head day) (in P2, P3, L1 and L2). Plasma total T3 concentrations were affected only by the sampling period, being significantly higher in the P1 period (0.96 ± 0.05 ng/ml) than in the dry period (0.72 ± 0.04 ng/ml) and in P3 (0.70 ± 0.02 ng/ml). In the C goats the plasma T4 concentration recorded in the P1 (65.6 ± 2.6 ng/ml) and L1 (62.8 ± 5.2 ng/ml) periods was significantly higher than that in P3 (40.2 ± 3 ng/ml). The variation in plasma T4 concentration in the H does was not significantly different throughout the different physiological stages. The decrease in plasma T3 and T4 concentrations in the second half of pregnancy in the H goats was delayed by 1 month. In addition, during the last 2 months of pregnancy, the higher concentrate diet induced higher plasma T3 (0.92 ± 0.06 ng/ml) and T4 (67.2 ± 6.6 ng/ml) concentrations when compared to the C (0.76 ± 0.04 and 45.1 ± 3.2 ng/ml for T3 and T4, respectively) group. The effect of energy intake on plasma thyroid hormone secretion during late pregnancy could be explained by a negative energy balance that usually occurs during this period. During lactation the higher energy diet had no significant effect on milk yield nor on the plasma thyroid hormone concentrations. The concentrate level of the C diet seemed to be sufficient to meet the energy requirements for milk production in these goats, as indicated also by the lack of body weight and body condition score (BCS) changes during lactation. The results of the present study highlight the relationships between thyroid activity and energy balance in the goat species. Circulating thyroid hormones represent a relevant metabolic index of the animals' nutritional state.