Serum thyroid hormone evaluation during transition periods in dairy cows

Abstract. Thirty-five multiparous Holstein dairy cows were selected from a high-producing dairy farm in northeastern Italy: 16 in second lactation (L2), 10 in third lactation (L3) and 9 in fourth lactation (L4). Blood sampling was carried out 7 ± 5 days before calving (Pre/C) and 7 ± 5 days after calving (Post/C). Serum triiodothyronine (T3), thyroxine (T4) and a thyroid-stimulating hormone (TSH) were assessed. Two-way analysis of variance (ANOVA) showed statistically significant effects of class (L2, L3, L4) and of period (Pre/C, Post/C) on the parameters studied (T3, T4, TSH). In particular, Bonferroni's multiple comparison test showed lower values in post-calving than in the pre-calving in L2 and L3 for TSH; lower values in post-calving than in the pre-calving in L2 for T3; and lower values in post-calving than in the pre-calving in L2, L3 and L4 for T4. Our results improve the knowledge of endocrine and metabolic changes occurring in dairy cows during transition periods and may be useful to supply a new strategy for the improvement of dairy cow farm management and reproductive performance.