Capacity of old versus young male rats to release thyrotropin (TSH), thyroxine (T₄) and triiodothyronine (T₃) in response to different stimuli

Abstract

Serum TSH, T. and T, were measured by radioimmunoassay (RIA) in 20–24 month old and 6–8 month old Long-Evans male rats during basal conditions, and upon exposure to cold temperature (4°C), injection of ovine TSH, thyroidectomy, and T₄ or T₃ treatment. Basal serum TSH values in the old and young rats were similar, but serum T₄ and T₃ levels in the old rats were significantly lower than in the young rats. No differences were observed in TSH release between old and young rats in response to cold temperature, but old rats showed no rise in serum T₄. TSH administration produced a significantly smaller increase in serum T₄ in old rats, but similar increases in serum T₃ in old and young rats. No differences were observed between old and young rats in the rise of serum TSH after thyroidectomy, or in the fall of serum TSH after T₄ or T₃ administration. These results are believed to indicate that the primary cause for reduced thyroid function in old rats lies in the thyroid gland itself.