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Effect of SSRI antidepressants on L-thyroxine requirements.

We read with interest the prospective study by de Carvalho et al. that evaluated the effects of fluoxetine or sertraline on thyroid indices in hypothyroid patients treated with lthyroxine (1). In this relatively small study, initiation of neither antidepressant altered l-thyroxine requirements. However, there is clearly a subset of patients in whom dose requirements are increased after starting sertraline, as we reported several years ago (2). We have also noted reductions in dose requirements when sertraline was stopped in some hypothyroid patients. This highlights the need for larger studies to establish the incidence of this relatively infrequent but significant interaction. Further study of the subgroup that requires thyroxine dose escalation when sertraline is started will be required to identify the genetic, environmental, or other predictors of whether or not sertraline will have an effect on a given patient’s thyroid hormone economy. In addition, many of the patients in our observational study were athyreotic on the basis of surgery or radioiodine therapy. We question the degree of hypothyroidism in the patients in the study by de Carvalho et al. (1). Perhaps no effect of sertraline was noted because their hypothyroidism was mild and partial, and the patients were able to compensate. Therefore, we ask the authors to provide doses of thyroxine in mcg per kg body weight as well as the underlying reason for the hypothyroidism.