235 ADMINISTRATION OF LEVOTHYROXINE IMPROVES THE PROPORTION OF TRANSFERABLE EMBRYOS FOLLOWING SUPEROVULATION IN JAPANESE BLACK CATTLE

Abstract

The thyroid gland has a major impact on the human female reproductive system. However, its influence on reproductive competence in livestock is unclear. In cattle, superovulatory treatments often lead to high numbers of ova/embryos but low numbers of transferable embryos. Levothyroxine is a synthetic thyroid hormone that is chemically identical to thyroxine (T4), and binds to T4 receptors found in follicle cells. We aimed to determine whether levothyroxine treatment (LT) could improve the proportion of transferable embryos after superovulation in Japanese black cattle (Wagyu). First, we observed associations between the concentrations of T4, free triiodothyronine and thyroxine, and superovulatory responses (n = 95). Blood was collected before superovulatory treatment by jugular venipuncture, and serum was separated and stored at –20°C until being assayed for thyroid hormones. Second, Wagyu cattle showing a low proportion of transferable embryos (<30%) were assigned randomly to an LT group (n = 10) or a control group (no LT, n = 15); the LT was administered by 3 i.m. injections of 2000 mg of levothyroxine given 3 days apart; Controls received no injection. On the last day of LT, hormonal stimulation for superovulation was started as described (Ideta et al. 2007 Cloning Stem Cells 9, 571–580). The proportion of transferable embryos was compared with the same individual before/after LT. Data were analysed using Student’s t-tests. There were no significant associations between the concentrations of thyroid hormones and the superovulatory responses (no. collected oocytes/embryos, or the proportions of transferable embryos or unfertilized oocytes). However, LT dramatically increased the proportion of transferable embryos in these low-responding Wagyu cattle (Table 1). In conclusion, administration of levothyroxine improved the proportion of transferable embryos following superovulatory treatment. Further studies will be needed to investigate how LT improves the quality of bovine embryos. Table 1.Effect of levothyroxine treatment on the proportion of transferable embryos after superovulation in Wagyu cattle (means ± SE)