Urinary kinetics of triiodothyronine and their modification with aging

The urinary kinetics of triiodothyronine (T3) in healthy young and old people has been studied. The behavior of urinary excretion is similar in both age groups, expressed in our mathematical model in the cumulative kinetics as well as in the urinary rate. However, a significant decrease (P less than 0.025) of renal clearance of T3 in elderly individuals has been found. No significant differences (P greater than 0.05) were observed in any other kinetic parameters. The possible influence of the reduction of glomerular filtrate has been discussed as the most important pathogenic factor of the decreased renal clearance of this hormone. Possibly, modifications of the distribution volumes are also involved. Due to the limited number of persons studied, for ethical reasons, the results obtained are not definitive.