Effect of I131 on Reproductive Performance in Ewes

The reproductive performance of seventeen ewes fed single doses of 3 mc I131 4–5 years ago was evaluated. The performance of these hypothyroid animals was compared with that of a group of euthyroid ewes of the same age kept under identical conditions. The estrus cycles, times in estrus during a 60-day period, and gestation periods of the hypothyroid ewes were not significantly different from those of the controls; however, there was greater variation within the hypothyroid group. The serum protein-bound iodine (PBI) values, determined prior to breeding and at intervals throughout and following the gestation period, indicated a slight rise at 50–100 days of pregnancy in both the experimental and control group. All of the control ewes (six/six) conceived after the first mating. Three of seventeen hypothyroid ewes failed to conceive or experienced early embryonic death, two of these three ewes manifested signs of estrus at regular intervals. Thyroid uptake of an I131 tracer dose in the hypothyroid ewes ranged from 5.6–20 per cent in those that were pregnant and 3–11 per cent in those that were not pregnant. The two ewes with the lowest thyroid I131 tracer uptake (5.6 and 6 per cent) died during the course of pregnancy. There were no cases of spontaneous abortion or still births in either the hypo- or euthyroid groups. The lambs from the hypothyroid group did not gain weight as rapidly during the first month of life and their PBI values at birth were lower than those of the control lambs.