Age dependent changes in iodine metabolism and thyroid function.

Absolute iodine uptake (AIU), serum thyroxine (T4), free T4 index and serum triiodothyronine (T3) were studied in 48 women and 63 men aged 20 to 90 years. AIU decreased with age in both sexes to values in the oldest age groups of about 60% of the values in the youngest age groups. AIU values for females were about 75% of the values for males. Free T4 index was found to be unaltered with increasing age but serum T3 decreased with age. These findings and the previous demonstrated decrease in T4 degradation rate seem consistent with the assumption that T4 and T3 production decreases with advancing age.