Influence of oestrogen on thyroid function in the ewe

1. Thyroid activation at oestrus occurs in several species, which may be related to oestrogen secretion occurring at the same time. In the ewe this relationship has been examined by measuring thyroid activity before and after oestrogen administration. 2. Measurements have been made of thyroid 131 I release rate and protein-bound 131 I concentration in the plasma in normal and ovariectomized ewes and of protein-bound 125 I in plasma from thyroid vein blood in ewes with exteriorized thyroid glands. 3. No evidence of an activation of the thyroid by oestrogen was observed in immature ewes or adult oestrous cycle, anoestrous or ovariectomized ewes, given oestrone or oestradiol dibenzoate in doses of 25-1000 μg by injection. 4. It was concluded that oestrogen may not be responsible for thyroid activation observed in normal oestrous ewes.