Abstract
Immunoreactive thyrotropin, extracted by affinity chromatography from the serum of Fischer 344 male rats and fractionated by gel filtration, is more polymorphic (in terms of molecular weight) in 22- and 30-month-old rats than in younger animals (3- and 12-month-old). Changes in hormonal levels with age involve thyrotropin and thyroxine but not triiodothyronine, serum thyrotropin being significantly decreased at 30 months and thyroxine at 22 and 30 months. Low serum thyroxine levels and increased thyrotropin polymorphism may suggest an impaired pituitary-thyroid axis in aged rats.
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