Thyroid function and aging: gender-related differences

The effects of aging on human or animal thyroid function are still not well defined. We evaluated some aspects of thyroid function during aging using an animal model (young and old Dutch-Miranda rats). In old rats of both gen ders, serum thyroxine (T4) decreased but serum thyrotrophin (TSH) remained unaltered, suggesting a disturbance in the pituitary-thyroid feedback mecha nism during aging. Serum tri-iodothyronine (T3) only decreased in old males , possibly because female rats are almost twice as efficient in hepatic T4 to T3 deiodination. Thyroidal T4-5'-deiodinase activity did not change much during aging, although it decreased slightly in males. Thyroidal iodothyro nine-deiodinase type I mRNA expression but not total thyroidal enzymatic ac tivity were higher in female than in male rats. Thus, ovarian/testicular ho rmones may modulate the expression and/or the activity of hepatic and thyro idal type I iodothyronine-deiodinase. Thyroperoxidase (TPO) and thyroglobul in (Tg) expression were higher in young male rats than in females. In males , TPO and Tg gene expression decreased with aging, suggesting that androgen s might increase their expression. Our results showed that aging induces re al changes in rat thyroid gland function and regulation, affecting at least pituitary, thyroid and liver functions. Furthermore, some of these changes were gender related, indicating that gonadal hormones may modulate thyroid gland function and regulation.