INTRODUCTION: Dimethyl fumarate (DMF) has been recently approved for first-line monotherapy of Multiple Sclerosis (MS). Its effects are due to mechanism modulating the immune system and activating antioxidative and neuroprotective pathways. PATIENT CONCERNS: A 59-year-old female patient affected by chronic Hashimoto's thyroiditis (HT) from 10 years was diagnosed with relapsing remitting MS in 2013. She started therapy with DMF in November 2016. DIAGNOSIS: After 2 months of therapy with DMF, the results of thyroid function test were abnormal. Thyroid ultrasonography confirmed the diagnosis of acute exacerbation of HT. INTERVENTIONS: This condition led to discontinuation of DMF therapy. OUTCOME: Two months after the interruption of DMF therapy, the findings of thyroid function test were within normal limits. CONCLUSION: The association of MS with autoimmune thyroid diseases supports a common immune-mediated pathogenic mechanism. We assume that the acute exacerbation of HT in our MS patient is associated not with the immunomodulatory effect of DMF but rather with its antioxidative mechanism.Constant monitoring of thyroid hormone levels should be recommended especially if the MS patients in treatment with DMF are affected by concomitant autoimmune thyroid diseases.
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