Thyroid hormones and their correlations with serum glucose, beta hydroxybutyrate, nonesterified fatty acids, cholesterol, and lipoproteins of high-yielding dairy cows at different stages of lactation cycle

The relationships between circulating thyroid hormones and serum glucose, beta hydroxybutyrate (BHB), nonesterified fatty acid (NEFA), cholesterol and lipoproteins of high-yielding dairy cows were studied in 125 adult Holsteins at various stages of lactation cycle: early (far-off) dry period (n = 24), late (close-up) dry period (n = 10), fresh cows (n = 22), early lactation (n = 13), mid-lactation (n = 27) and late lactation (n = 29). Decreased levels of thyroxin (T₄) and triiodothyronine (T₃) were noted in peripartum cows which were extended to mid- and late-lactation cows, respectively. In fresh cows, glucose showed correlations with T₄ (r = 0.619, P < 0.01) and T₃ (r = 0.627, P < 0.01). In the same cows there were correlations between T₄ and BHB (r = 0.590, P < 0.01) and NEFA (r = 0.470, P < 0.01). In late dry cows, free thyroxin (fT₄) showed correlations with triglyceride (TG) and very-low-density lipoprotein (VLDL; r = −0.638, P < 0.05). Pooled data of the various stages of lactation cycle revealed correlations between glucose and T₄ (r = 0.20, P < 0.05), glucose and T₃ (r = 0.395, P < 0.01), cholesterol and T₃ (r = −0.201, P < 0.05), and free triiodothyronine (fT₃) and high-density lipoprotein (HDL; r = 0.178, P < 0.05). It appears that the time and the pattern of changes and/or correlations of serum thyroid hormones and lipid fractions may vary among dairy cows and other animals.