Relationship Between Prolactin and Thyroid Hormones in Insulin-Resistant Dairy Cows
Insulin resistance can compromise metabolic pathways in dairy cows. This phenomenon may affect metabolic hormones such as prolactin and thyroid hormones. Twenty-five multiparous Holstein dairy cows were divided into five equal groups containing early, mid and late lactations, far-off and close-up dry periods. All cows received dextrose 50% intravenously at 500 mg/kg, 10 ml/kg/hr. Blood samples were collected from all animals prior to and 1, 2, 3 and 4 hr after dextrose 50% infusion and sera were separated to determine the levels of glucose, insulin, prolactin and thyroid hormones containing T3 and T4. Insulin resistance phenomenon was detected in early and mid lactation and close-up dry periods. There were positive correlations between thyroid hormones and prolactin in the early lactation group, but these relationships were negative during mid lactation. Regarding the effectiveness of prolactin and thyroid hormones on the metabolism and lactogenesis of dairy cows, it may be stated that they can directly affect metabolism enhancement in different situations such as insulin resistance. These findings provide some clues on the relationship between prolactin and thyroid hormones in insulin-resistant Holstein dairy cows.
Key words: insulin resistance, prolactin, thyroid hormones, dairy cow