Free thyroxine estimation for the screening of hyper- and hypothyroidism in an adult population.

Serum free thyroxine (FT4), total T4 (T4) and total T3 (T3) were determined by radioimmunoassay in 1,114 adults during a periodic health evaluation to detect unsuspected thyroid dysfunction. As a result, 3 patients with hyperthyroidism, 4 with hypothyroidism, 3 taking thyroid medication, 4 with chronic thyroiditis, 1 with simple goiter and 3 women under estrogen administration were found. Free T4 values were within the normal limits in the present four subjects taking estrogens, but all of them showed high T4 and one high T3 values. FT4 and T4 were low in four patients with hypothyroidism, but two of them showed normal T3 values. For the screening of hyperthyroidism, FT4 was the most reliable measurement and determination of either FT4 or T4 was suitable for the screening of hypothyroidism, but T3 measurement did not diagnose all patients with hypothyroidism, but T3 measurement did not diagnose all patients with hypothyroidism.