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

Serum TSH, T4 and T3 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 degrees 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 whowed 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.