Effect of Advancing Age on Thyrotropin Content of the Pituitary and Blood Of the Rat.

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Abstract
The thyroid hormone secretion rate (TSR) of female Sprague-Dawley-Rolfsmeter rats has previously been shown to decline markedly from weaning time to about 4 months of age. In the present study it was shown that the pituitary and plasma levels of TSH were low at weaning time and gradually increased until 80 or 95 days of age, then showed a slight decline. In order to explain the observations of a declining TSR associated with an increase in plasma TSH, it has been suggested that: the sensitivity of the thyroid gland to TSH decreses with age; the t1/2 of TSH may decrease with increasing age; the secretion of L-T3 [triiodothyronine] in the immature rat may be high and decline with age.