The authors studied thyroid function by dosage of total T3 and T4, corresponding free portions, thyroid binding globulin (TBG), thyroid stimulating hormone (TSH) and reverse T3 (rT3) in 68 clinically euthyroid patients, treated with amiodarone, divided in two groups including 25 subjects treated for less than one month and 43 subjects in therapy for more than one month. The data obtained in the first group have been compared with the second one and with data obtained in a group of 40 healthy subjects. The statistical analysis of results showed a reduction of total T3 and free T3 and an increase of TSH, total T4, free T4 and rT3 in each of two groups of treated patients compared with control group, while there was no difference in comparison between the two treated group.