Reproductive patterns in cyclic and pregnant thyroidectomized mares

Three Quarter-horse mares were thyroidectomized at about 1.5 years of age. Three similar intact mares served as controls. The study continued through two breeding seasons. The thyroidectomized mares were lethargic, rear limbs were oedematous and hair coats were coarse. They displayed a tranquil oestrous behaviour when exposed to a stallion and were only mildly antagonistic when not in oestrus. Length of oestrous cycles varied but most often they were 19-24 days long. Duration of oestrus (mean +/- s.e.m.) for the control and thyroidectomized mares was 12.9 +/- 2.9 and 11.7 +/- 2.2 days respectively (P greater than 0.05). The peak of LH during oestrus was as high as 60 ng/ml blood serum with no difference between the two groups. Peak progestagen on Day 7 after ovulation for controls was 9.0 +/- 1.6 ng/ml and was not different (P greater than 0.05) from the peak of 6.3 +/- 1.7 ng/ml for thyroidectomized mares on Day 8. Pregnancy was achieved in both groups of mares, including the use of semen from a thyroidectomized stallion. Thyroxine was detectable in one pregnant thyroidectomized mare during the last two-thirds of pregnancy only.