Thyroid function was examined periodically in a group of 85 patients treated with low-dose amiodarone, and followed for 1-13 years (mean 3.6). Biochemical hypothyroidism (elevated thyroid stimulating hormone (TSH) only) developed in 8 patients and clinical hypothyroidism in 3, while hypothyroidism developed in 5. In the first 11 cases amiodarone was continued but 1-thyroxine was added. In the 5 that became hyperthyroid, amiodarone was stopped and thyroid function became normal within a few months. In the 69 patients without thyroid dysfunction, elevated thyroxine (T4) and free thyroxine index (FTI) were found in 20% and 17%, respectively, and elevated free T4 (FT4) in 28%; all had normal total tri-iodothyronine (TT3) and TSH. In practically all patients, reverse T3 (rT3) rose more than 30% above initial levels. It is concluded that in patients treated with low dose amiodarone: 1) thyroid function should be followed closely; 2) elevated levels of TT4, FTI and FT4 do not necessarily imply hyperthyroidism; 3) hypothyroidism may be biochemical only, without progression to overt hypothyroidism, despite continued treatment; 4) both the hypo- and the hyperthyroidism induced are usually reversible; 5) amiodarone may be continued despite onset of hypothyroidism, provided replacement therapy is given.