Pituitary thyrotroph hypofunction in aged euthyroid subjects as assayed by the administration of iopanoic acid.

Iopanoic acid, an iodine containing contrast medium was administered orally to healthy subjects aged 20-40 years (n = 9) and 70-90 years (n = 10) and also to sick aged persons (aged 70-90 years, n = 10). Thyroid hormones: T4, T3, rT3, T3-uptake and thyrotropin (TSH) serum levels were estimated before and three and seven days after iopanoic acid. No significant TSH increase could be seen in the old-age groups: the T4 increases were similar in all three groups as well as the slight T3 decreases; the serum rT3 increase was significantly greater in the aged healthy and the sick subjects. The data suggests that in aged humans serum T4 elevation after iopanoic acid is a result of delayed thyroxine metabolism and not secondary to TSH release. Iopanoic acid administration is, aside from the TRH test, another model to demonstrate in geriatric patients thyrotroph hypofunction via the altered effect of inhibition of T4 deiodination on TSH release.