The objective of this study was to assess the association between thyroid hormone profile and resumption of postpartum ovulations in cattle. Lactating Holstein cows (n=40, milk yield 38.51±1.23 kg/d) were selected and based on the ultrasound results, blood estradiol and progesterone analysis, were divided into two groups. The cows were classified into ovulatory (AO) group if the first ovulation did not occur until 45 days after calving (n=16) and ovulatory group (O) if the first ovulation occurred 45 days or less after calving (n=24). Blood samples were collected from the cows weekly from day 21 to 48 postpartum to evaluate thyroid hormone levels and blood parameters. Results showed that milk yield and glucose concentrations did not differ between the groups. Serum estradiol and progesterone conentrations in ovulatory cows were significantly higher than those in AO cows. Cows in O group had better condition with respect to days to first service (DFS), open days (OD), days to first ovulation (DFO) and conception rate than cows in AO group. Thyroid hormone analysis show that T4 concentration and T4:T3 ratio in the AO group were significantly higher than those in O group (P=0.001), while there were no statistically significant differences between groups for T3 concentrations. Also, T3 and T4 concentrations and T4:T3 ratio in both groups were not affected by time (P=0.17) and time×group interaction (P=0.25). In addition, no significant difference was found in the non-esterified fatty acids (NEFA) concentration between the two groups. Unlike NEFA, beta-hydroxybutyric acid (BHBA) levels in AO group were significantly higher than those in O group (P=0.01). In conclusion, cows with different ovarian activity postpartum had different thyroid hormone profile. Increased T4 concentration and T4:T3 ratio was associated with delay in ovulation and resulted in greater day to first service and open days and followed that lower conception rate
