HYPOTHYROIDISM AFFECTS REPRODUCTIVE REFRACTORINESS AND THE SEASONAL ESTROUS PERIOD IN WELSH MOUNTAIN EWES

Welsh Mountain ewes (n = 6) were rendered hypothyroid by daily treatment with methylthiouracil (35 mg/kg), beginning in early August and ending in late February. Plasma thyroxine levels were reduced by mid-September to about 33% of those in untreated ewes (n = 6). The two groups of ewes were held under natural daylengths until 5 October, then on 12 h light: 12 h darkness (12L:12D) until 28 February when the photoperiod was reduced to 8L:16D. The onset of reproductive cyclicity in October was similar in both groups of ewes but the end of the reproductive period occurred later (P < 0.05) in the hypothyroid ewes (29 January ± 7 days (S.E.M.)) than in the untreated controls (6 January ± 7 days). As a result, the duration of the seasonal reproductive period was significantly (P < 0.05) longer in the hypothyroid (122 ± 9 days) than in the untreated ewes (91 ± 10 days). The number of oestrous cycles (duration 15.4 and 15.7 days in the hypothyroid and untreated ewes respectively) was 7.0 ± 0.6 in the hypothyroid ewes and 5.0 ± 0.5 (P < 0.05) in the normal ewes. Reducing the photoperiod overcame the reproductive refractoriness and anoestrus in both groups, the hypothyroid ewes beginning to cycle on 13 April (± 0.5 days) after an anoestrous period of 72.8 ± 7.1 days. The untreated ewes began to cycle 2 weeks later on 26 April (± 1.7 days) after an anoestrous period of 112.0 ± 8.5 days (P < 0.001). Hypothyroidism appears, therefore, to affect the development of the refractoriness which ends the reproductive season in the sheep as well as its intensity. Such effects are similar to those found after full thyroidectomy but less pronounced since thyroidectomy is able to suspend anoestrus completely.