Clinical and biochemical indices of thyroid function were compared in 18 euthyroid control subjects and 54 patients at risk of developing hypothyroidism, and the assessment repeated after a four-week trial of oral triiodothyronine (T3) 40 microgram daily. Patients with minor elevation of the pre-treatment serum TSH concentration (5-15 microU/ml) showed significant reduction in the free thyroxine index (FTI), prolongation of the basal ankle reflex half-relasation time (ART), and shortening of the ART after oral T3 treatment, when compared either to control subjects or to patients with a normal TSH level. It is concluded that patients with minimal TSH elevation have minor but parallel abnormalities of both thyroid hormone concentration and action.