Serum concentrations of the iodothyronines in elderly subjects: decreased triiodothyronine (T3) and free T3 index.

In four groups of subjects free of thyroid disease, the following determinations were made: serum concentrations of thyroxine (T4), triiodothyronine (T3), reverse T3(rT3) and diiodothyronine (T2), and calculated indices of free thyroxine (FT4) and free triiodothyronine (FT3). Group A comprised healthy subjects aged 16-64; Group B, 24 healthy elderly subjects aged 68-95; Group C, 23 elderly patients with mild well-controlled chronic illnesses, aged 70-85; Group D, 40 nursing home residents aged 66-100. Serum T4 and T2 concentrations and the FT4 index were not affected by age; the rT3 concentration was slightly but significantly elevated only in Group D patients. Serum T3 concentration was significantly lower in all groups of elderly subjects and decreased FT3 index measurements were detected after age 75. It was concluded that old age, without complicating illness, is accompanied only by a decrease in the serum level of T3 and the FT3 index; values for other iodothyronines are unchanged. Clinicians should consider the age-related changes in T3 and FT3 values when interpreting thyroid function tests.