Serum levels of TSH, T3 and T4, pituitary responsiveness to TRH, and T3-resin uptake were examined in aging male and female rats. Despite normal serum TSH levels, serum T3 and T4, levels were significantly reduced in old male rats (19-26 months of age) when compared to young (120-150 days of age) and middle-aged males (12-18 months of age). In old female rats, serum TSH and T3 levels were not significantly (p >0.05) different from those of young or middle-aged females. Serum T4 levels were significantly lower in old females when compared to younger rats. The To-resin uptake of old rats was not significantly different from the value of young rats. Serum T4 levels, 3 hours after TRH induced TSH release, were significantly depressed in old male and female rats. Pituitary sensitivity to TRH was unaltered in old rats. These findings suggest that thyroidal release of T4 and T3 is depressed, and that the hypothalamic-pituitary axis is insensitive to diminished thyroid hormone feedback in old rats.
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