Serum thyroid hormones and performance of offspring in ewes receiving propylthiouracil with or without melatonin

Two experiments were conducted during mid-gestation to examine effects in ewes of propylthiouracil (PTU) treatment alone or with melatonin on serum thyroid hormones, postpartum reproduction, and lamb performance. In the first experiment, beginning on day 0 (first day of treatment when all animals were 72.2 ± 0.9 days of gestation), ewes received daily treatments (gavage) consisting of either 0 mg (n = 6) or 40 mg (n = 6) PTU/kg body weight/day for 15 days. After 15 days, the 40 mg dosage was decreased to 20 mg/kg body weight for an additional 20 days (35 days of PTU). Serum thyroxine (T4) did not differ (P > 0.10) between groups through day 4; but on day 5, control females had a serum value of 67 ng/ml compared with 46 (±5) ng/ml for PTU-treated ewes (P = 0.02). On the last day that 40 mg of PTU was administered, serum T4 averaged 67 and 7 (±5) ng/ml (P < 0.001) in the two respective groups. Serum T4 remained low and was 80 and 1 ng/ml (P < 0.001) in control and treated ewes on day 34. Serum T4 rose gradually after PTU but remained different from that observed in control ewes through day 48. Lambs from control and treated ewes had similar (P = 0.46) T4 values at birth but lambs from PTU-treated ewes had lower (P = 0.03) birth weights than did those from control ewes. Serum progesterone (P4) after parturition indicated a lack of cyclicity in all ewes. In the second experiment, beginning on day 0 (76.8 ± 4.7 days of gestation), ewes received PTU as in Experiment 1. In addition, after 15 days of PTU, melatonin was given (i.m. injections at 5 mg/day) for 30 days. Propylthiouracil decreased (P ≤ 0.05) serum T4 on days 25 through 41 of the treatment period. Serum T3 was lower (P < 0.05) in PTU-treated ewes from day 8 through the end of PTU administration. Postpartum P4 was below 1 ng/ml on all days indicating acyclicity. Lamb body weight and serum IGF-I were similar (P > 0.60) for lambs born to control and treated ewes. Female offspring of PTU + melatonin-treated dams reached puberty, became anestrus, and returned to cyclicity at similar (P > 0.10) times to contemporary ewe lambs. Results indicate that 40/20 mg PTU alone or with melatonin does not induce cyclicity after lambing in spring lambing ewes and has little effect on offspring performance.