Correlation of serum levels of T3 and T4 during the dry and postpartum periods with ovarian rebound in primiparous and multiparous cows

The thyroid gland is one of the major endocrine glands which plays an important role in vital balance of the body by secreting two hormones, T3 and T4. Because effects of these two hormones affect the activity of many body organs, in this survey the effects of these two hormones on the return of ovarian activity in Holstein cows were examined. For this purpose, 60 primiparous cows (having one pregnancy) and 60 multiparous (having two or more pregnancies) were considered for this survey. In both groups, the blood samples were taken 10 days before parturition and 10 to 20 days after parturition.  After centrifugation and serum separation, samples were stored at -20 o C. Afterwards in laboratory, T3 and T4 values were measured by using ELISA kit. The results indicate that the values of T3 and T4 in primiparous cows in the prenatal and postpartum period were more than multiparous cows (p<0.05). In both groups, values of T3 and T4 in postpartum period were reduced compared to prenatal period. T4 values in multiparous cows in the postpartum period showed significant correlation with the return time of ovarian activity. Finally, it can concluded that decreasing thyroid function and thyroid hormones deficiency, particularly T4, in the postpartum period can influence the prolongation of the onset of ovarian activity in multiparous cows but in primiparous cows due to the sufficient activity of the thyroid gland, the onset of ovarian activity should be sought in other factors.