THYROXINE TURNOVER IN EUTHYROID MAN WITH SPECIAL REFERENCE TO CHANGES WITH AGE

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Abstract
Thyroxine turn-over has been studied in 73 euthyroid male subjects ages 18 to 91. Thyroxine distribution space (TDS) and fractional turnover rate (k) were measured following intravenous administration of thyroxine-I131. Both of these parameters were found to decrease with age (p < .001). Daily thyroxine degradation calculated as the product of TDS x k x plasma protein-bound iodine (PBI) decreased linearly by about 50 per cent over the age span measured (89 [mu]g thyroxine I/day, mean age 21; 44 pig/day, 9th decade; r = -0.69; p < .001). The PBI does not change with age. Since for the steady state degradation equals output, the findings are interpreted to indicate a decrease with age of thyroxine output, possibly due to an age dependent decrease in the enzyme system(s) which degrade thyroxine, with homeostatic maintenance of an unchanged level of circulating hormone.