Thrombocytopenia after Long-Term Use of Low-Dose Methimazole.

Abstract

Background: Methimazole (MMI) is an antithyroid drug and can cause hematologic toxicities. Isolated thrombocytopenia due to MMI has been reported very rarely. Here we present a patient with isolated thrombocytopenia that developed as a result of long-term treatment with low-dose MMI.

Methods: A patient with hyperthyroidism who had been taking low-dose MMI for 8 years was referred to our clinic with thrombocytopenia. His platelet count was 55 x 103/mm3. He had no leukopenia. The patient was euthyroid, no other etiology for the thrombocytopenia was identified and MMI was discontinued.

Results: The patient's platelet count increased after discontinuation of MMI. 3 weeks after discontinuation of MMI, his platelet count was 112 x 103/mm3. Drug-induced thrombocytopenia was assumed as no other etiology was detected and platelet count increased after drug discontinuation.

Conclusions: Although MMI has been reported to cause thrombocytopenia and bleeding, isolated thrombocytopenia due to MMI is quite rare in the literature. This case shows the development of thrombocytopenia in a patient treated with low-dose MMI. Since the patient was euthyroid and this side effect occurred after long-term use of low-dose MMI, it can be assumed that it is a direct toxicity due to dose accumulation of the drug over years or due to immunological mechanisms.