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

Serum free thyroxine (FT4) was determined in 1,114 adults (male 239, female 875) in a periodic health evaluation in 1980 to detect unsuspected thyroid dysfunction, especially hyper- and hypothyroidism. The participants were dwelling in two towns of Miyagi prefecture. Beside FT4, serum T4 and T3 were also determined by radioimmunoassay. If thyroid dysfunction was suspected, further detailed examinations such as TRH-test (500 micrograms i.v.), radioimmunologic determinations of serum TSH and TBG, resin-sponge T3-uptake, 24-hr thyroid radioiodine 131I-uptake, radioiodine thyroid scan and anti-thyroid antibodies were performed. There were 3 patients with hyperthyroidism (0.27%), 4 with hypothyroidism (0.36%), 3 taking thyroid medication (2; Hashimoto's disease, 1; goiter), 3 on estrogen administration, 4 with Hashimoto's disease and 1 with goiter. Excluding these 18 patients, FT4, T4 and T3 values in 1,096 euthyroid subjects, 236 males and 860 females, were 1.1 +/- 0.3 (mean +/- S.D.), 1.1 +/- 0.3 and 1.0 +/- 0.3 ng/100 ml, 8.9 +/- 1.5, 8.8 +/- 1.6 and 9.0 +/- 1.5 micrograms/100 ml, and 122 +/- 33, 125 +/- 26 and 122 +/- 35 ng/100 ml, respectively. Serum FT4, T4 and T3 showed the distribution of logarithmic normal probability. The 95% normal range for free T4 was 0.60 to 1.80 ng/100 ml, total T4 6.0 to 11.8 micrograms/100 ml, and T3 84 to 176 ng/100 ml, respectively. Out of 1,114 subjects examined, the cases to be reexamined for the higher serum concentration than normal were 26 in FT4, 35 in T4 and 27 in T3, respectively. And the cases for lower values were 28 in FT4, 31 in T4 and 24 in T3, respectively. Serum FT4 values in the subjects during the administration of estrogens were within the normal range. FT4 and T4 were low in four patients with hypothyroidism, but two of them showed normal T3 values. Determinations of serum FT4, total T4 and total T3 were all useful for the screening of hyperthyroidism. But serum FT4 was the most reliable of the three. Determination of either serum FT4 or total T4 was suitable for the screening of hypothyroidism, but serum total T3 measurement did not cover all patients with hypothyroidism.