Effect of amiodarone on L-triiodothyronine stimulation of [3H] thymidine incorporation into GH3 cells.

The antiarrhythmic agent amiodarone was found to inhibit the stimulatory effects of L-triiodothyronine on [3H] thymidine incorporation into GH3 rat pituitary tumor cells. This inhibitory effect of amiodarone was detected at concentrations as low as 0.5 microM; at 2 microM greater than 50% of the stimulatory effect of L-triiodothyronine was inhibited. The effect of amiodarone was present at all concentrations of L-triiodothyronine tested (50 pM to 10 nM), suggesting that amiodarone acted as a non-competitive antagonist. These studies raise the possibility, therefore, that the effect of amiodarone on thyroid hormone metabolism may be mediated in part by an inhibition of thyroid hormone action at the cellular level.