Influence of stage of lactation and season on thyroid hormones and milk yield of Holstein cows

Holstein dairy cattle were maintained to determine the effect of stage of lactation and season of the year on serum and milk levels of thyroid hormones as measured by radioimmunoassay.(RIA) . Each stage of lactation (early, mid and late lactation) was represented by 24 cows and each season (fall, winter, spring and summer) was represented by 18 cows. Serum (T4,T3 and rT3 ) concentrations were significantly higher in mid and late lactation than those in early lactation . Serum T4 concentration was significantly higher (p ≥ 0.01) in fall and winter than in spring and summer (53 vs. 32 ng/ml). Serum T3 concentration was also higher in fall and winter than in spring and summer (5.3 vs. 2.9 ng/ml). Reverse T3 levels were significantlyhigher (p ≥ 0.01) in fall than in the other three seasons. They also were significantly higher in winter than in summer. The trend from fall to summer was downward in the four seasons (272, 242, 237 and 213 pg/ml). Milk T4 concentration was higher in mid lactation and lower in early and late lactation. Milk T4 concentration showed significant differences (p ≥ 0.01) among fall, winter and spring, but no difference between spring and summer. Milk T3 concentrations were significantly different (p ≥ 0.01) among fall, winter and spring. These data suggested that concentration of thyroid hormones in blood and milk may be depending on the environmental temperature and stage of lactation.