Abstract. In order to test the reactivity of TSH to TRH during amiodarone treatment we investigated 7 hypothyroid subjects treated with 50 μg T3/day. A TRH test (200 μg iv) was performed before and after 6 weeks of treatment with 400 mg amiodarone/day. Amiodarone treatment induced a significant increase in serum total and free T3 (from 2.17 ± 0.13 to 3.55 ± 0.58 nmol/l and from 5.6 ± 0.61 to 9.46 ± 1.41 pmol/l). Basal TSH levels were significantly decreased and the maximal stimulation of TSH 20 min after TRH injection was only 20.0 ± 3.3 mU/l during amiodarone treatment compared with 61.4 ± 10.4 mU/l before treatment. These results indicate that in hypothyroid patients treated with amiodarone, the TSH response to TRH is blunted and that this is likely to be related to the higher total and free T3 levels or to a direct effect of amiodarone at the pituitary level.