Serum free thyroid hormones in subclinical hypothyroidism.

Serum free thyroxine (FT4) and free triiodothyronine (FT3) concentrations were measured in a group of 52 patients with subclinical hypothyroidism (SH) and in an equal group of age and sex-matched normal controls. SH was defined by normal total T4 (TT4) and total T3 (TT3) concentrations, normal FT4 and FT3 indices, raised TSH levels, in the absence of signs and symptoms of hypothyroidism. Serum FT4 levels averaged 6.1 ± 1.6 pg/ml (mean ± SD, p < 0.001 vs controls), with values below lower normal limits in 33/52 patients; mean FT3 concentrations averaged 3.1 ± 0.7 pg/ml (p < 0.001 vs controls), with values below lower normal limits in 8/52 patients. The analysis of results by the Galen and Gambino predictive value model demonstrated a higher sensitivity, but a lower specificity of FT4 as compared to FT3 in the diagnosis of SH. These results indicate that FT4 should be measured in addition to TSH for the diagnosis of impending thyroid failure, thus showing that in many cases patients with so-called subclinical hypothyroidism are actually already mild hypothyroid.