The activity of the thyroid gland is highest during oestrus in regularly cyclic adult female rats, and it has been suggested (Brown-Grant, 1962) that this may be due to a period of hypothalamic stimulation of pituitary TSH secretion during prooestrus. The release of luteinizing hormone that leads to ovulation can be blocked by a suitably timed injection of 'Nembutal' (sodium pentobarbitone) during the day of prooestrus and if the increase in thyroid activity were also inhibited this would provide additional evidence in favour of this concept.
Rats were kept under the same conditions and the vaginal cycles followed in the same way as in earlier studies (Brown-Grant, 1962). Thyroid activity was measured by determining the 2·5 hr. uptake of 131I by the gland on the morning of the predicted day of oestrus. The occurrence or failure of ovulation was determined by the examination of serial sections of the ovaries.
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