Levothyroxine therapy and imatinib.

Levothyroxine requirements in patients with hypothyroidism are usually stable. Consequently, thyroid function is usually monitored once or twice yearly. Occasionally, the dose of levothyroxine can be changed by pharmacological reactions. We report the case of a 59-year-old woman who was under levothyroxine therapy for hypothyroidism secondary to subtotal thyroidectomy, with clinical and biochemical euthyroidism, who required an increased dose of levothyroxine after starting imatinib therapy. The patient was diagnosed with chronic myeloid leukemia and imatinib therapy was started. Subsequently, we observed clinical and biochemical hypothyroidism, requiring an increase in levothyroxine dose. Some cases of hypothyroidism after initiation of imatinib therapy in patients with levothyroxine replacement therapy have recently been published. Our case provides further evidence of a reaction between the two drugs. Therefore, we discuss the most likely physiopathological mechanisms contributing to imatinib-induced hypothyroidism in patients under levothyroxine replacement therapy.