We have studied 18 patients with low serum thyrotrophin (TSH) levels accompanied by normal thyroxine (T4) and triiodothyronine (T3) levels in order to clarify whether or not they are thyrotoxic. Serum T4, T3, free T3, free T4 and TSH were estimated three times weekly for 3-4 weeks. Thyroxine binding globulin (TBG) levels were estimated on one occasion only. Thyroid hormone data were expressed as the cumulative probability of having at least one result above the relevant normal range by the nth sample (Pn). For free T4 this probability was 61% by the 10th sample. Free T4 values were confined to the upper half of the normal range or above throughout the study. In contrast, free T3 values were distributed evenly within the normal range (P10 = 12%). For total T4 and total T3, P10 was 34 and 11% respectively. Thus, subjects with subnormal TSH levels but normal T4 and T3 levels have been shown to have elevated serum free T4 levels, an indicator of biochemical hyperthyroidism.