Serum and pituitary TSH and response to TRH in developing male and female rats.

Serum and pituitary thyroid-stimulating hormone (TSH) was measured by radioimmunoassay in male and female Sprague-Dawley rats from 21 through 80 days of age. In males, serum TSH levels increased progressively from days 30 through 50, and were found to be lower on days 60, 70, and 80. In females, serum TSH levels were elevated on days 40 and 50, compared with day 30, but declined on days 60-80. A sex difference in serum TSH levels, with those of the male higher than those of the female, appeared by day 30 and was maintained through day 80. The anterior pituitary (AP) content of TSH in males increased from days 21 through 50 and remained constant through day 80; in females the AP content increased between days 25 and 60 and remained constant through day 80. In males, thyrotropin-releasing hormone (TRH) induced a significant elevation in serum TSH at all ages tested, but was less effective in increasing serum TSH on day 25 than on days 15 or 40 or in 3-4-month-old rats. The response to TRH appeared to be sustained longer in adults than in all other age groups. These observations indicate that serum TSH levels increase in both male and female rats at about the time of puberty and then decline, and that changes in the response to TRH may account in part for the increase in serum TSH levels during development.