[The effect of amiodarone on thyroid gland function]

Plasma concentrations of T4, FT4, T3, rT3 and TBG, as well as of TSH before and after stimulation with TRH were studied in 25 patients, who had been treated with amiodarone for up to nine years. At the beginning of therapy, all the parameters mentioned above were found to be in the normal range in all patients. After two months of therapy, T4 had increased from 100 nmol/l +/- 24 nmol/l to 155 nmol/l +/- 32 nmol/l (p less than 0.01), and FT4 from 22 pmol/l +/- 10.5 pmol/l to 32 pmol/l +/- 8 pmol/l (p less than 0.01). T3 had decreased to the lower normal range (n.s.). TBG showed no significant changes. The TRH-tests had been normal in the beginning, but they remained positive in only 20% of the cases. At the end of the study, rT3 exceeded the normal range in all 25 patients. Two patients developed definite hyperthyroidism with elevations of T3 up to 4.7 nmol/l and 7.5 nmol/l, respectively. In one of them, we decided to discontinue amiodarone. Testing of thyroid function under antithyroid drug therapy revealed a hyperfunctioning autonomous adenoma, which was successfully eliminated by radioactive iodine therapy. In the other patient, it was not possible to withdraw amiodarone, so we initiated long-term treatment with antithyroid drugs. Our data support the assumption that amiodarone causes an impairment of the peripheral conversion of T4 to T3. As a result, one finds elevated serum concentrations of T4, which, in combination with the mainly negative TRH-test, must not be interpreted as proof of a hyperthyroid metabolic state being present. Hyperthyroidism is confirmed only if serum concentrations of T3 are also elevated.