BIOLOGIC HALF-LIFE OF I131IN THE THYROID OF HEALTHY MALES*

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Abstract
The biologic half-life of I131 in the thyroid gland was measured in 38 healthy males ranging in age from 22 to 83 years. In subjects 50 years of age and over the biologic half-life of thyroidal I131 was directly proportional to the age of the subject and inversely proportional to the initial 24-hour percentage uptake in the gland. In subjects less than 50 years of age the biologic half-life was more or less independent of the age of the subject and of the 24-hour percentage uptake of I131. Variation in the re-accumulation of I131 may explain this difference between young and old subjects. The results were compatible with the concept of a progressive decrease in secretion of thyroid hormone due to decreased peripheral utilization with advancing age.