A comparison of triiodothyronine and thyroxine in the potentiation of tricyclic antidepressants.

Several studies suggest that small amounts of triiodothyronine (T3) will convert tricyclic antidepressant nonresponders into responders within a few days to a few weeks. As thyroxine (T4) is converted to T3 to have its physiological effect, it has been assumed that T4 would have a similar antidepressant effect to that of T3. We carried out a randomized, double-blind evaluation of the antidepressant-potentiating effect of T3 as compared with T4. Significantly more patients responded to 3 weeks of T3 as compared with T4 potentiation. Small but significant differential effects of the two thyroid hormones were noted on the Hamilton Rating Scale for Depression.