Combination Levothyroxine and Levotriiodothyronine Therapy for Hypothyroidism Treatment-Is it Worth the Risks?

About 4.6% of the U.S. population ages 12 and older and 3.05% of European population has hypothyroidism [1,2]. Thyroid produces thyroxine (T4) and triiodothyronine (T3) but T3 is more active at the cellular level because of its higher affinity for the nuclear thyroid hormone receptors. In humans, approximately 80% of the T3 produced daily derives from monodeiodination of T4 in extrathyroidal tissues [3]. Current guidelines consistently recommend Levothyroxine (LT4) monotherapy as treatment of choice for hypothyroidism management [4,5]. The goal of therapy is to restore physical and psychological wellbeing and normalize serum TSH. Although LT4 monotherapy is effective, up to 5-10% of hypothyroid patients with normal TSH on LT4 does not feel entirely well and reports persistent symptoms [5]. Given the high prevalence of patients who are not satisfied with LT4 monotherapy, a review of the literature evaluating efficacy and safety of combination levothyroxine and levotriiodothyronine therapy (LT4/ LT3) is worthwhile.