IMMUNOHISTOCHEMICAL AND RADIOIMMUNOLOGICAL ASSESSMENT OF THYROTROPHS IN THE PITUITARY OF AGING RATS

Thyrotrophs were studied by quantitative immunohistochemistry in the p ituitary gland of young (4 months), old (20 months) and very old (29 m onths) male rats. An attempt was also made to correlate morphometric p arameters with serum levels of thyrotropin (TSH), thyroxine (T-4) and triiodothyronine (T-3). Cells were immunostained by the peroxidase-ant iperoxidase method and hormones were measured in serum by specific rad ioimmunoassays. There was a marked age-related reduction in TSH cell n umber, volume density and surface density but a significant increase i n TSH cell area and perimeter. Basal serum levels of TSH increased, T- 4 decreased and T-3 remained unchanged with age. There was a highly si gnificant (p < 0.001) negative correlation between serum TSH and T-4, but no significant correlation was found between TSH and morphometric parameters. The present results suggest that aged rats possess a reduc ed but functionally preserved thyrotrophic cell population. The coexis tence of high circulating levels of TSH with reduced serum T-4 suggest s that aging brings about a progressive desensitization of the thyroid to TSH.