Total and free thyroid hormones (T3, T4, FT3 and FT4), TSH and its response to TRH were determined in sixty-three patients affected by autonomous thyroid nodules: mean concentrations of free T3 (FT3) were significantly higher in hot nodules (suppressing extranodular tissue on the scan) as compared to warm ones, even in those cases where total T3 and T4 were within normal ranges (hot nodules, group as a whole: 8.8 +/- 3.5 pg/ml; warm nodules: 5.3 +/- 1.2; hot nodules with normal total T3 and T4 concentrations: 7.5 +/- 3). Also the clinical condition of thyrotoxicosis appeared to be correlated with FT3 concentrations (toxic patients, group as a whole: 9.6 +/- 4.0 pg/ml; euthyroid patients: 6.8 +/- 3.1; toxic patients with normal values of T3 and T4; 8.3 +/- 2.8). On the contrary the correlation of total and free thyroid hormone concentrations with the response of TSH to TRH was not significant.