Although it is well known that aplastic anemia and agranulocytosis are potential lethal adverse reactions of antithyroid treatment, we present a case of methimazole administration in a patient with bone marrow transplant for multiple myeloma, with favorable evolution. We present the case of a 43 y.o. male, known with Graves disease since 2010 (on ATS treatment for only 6 months), vitiligo, systemic sclerosis and type 1 diabetes, diagnosed with multiple mieloma Ig G Tipe, Stage III C in 2017. He received treatment with Velcade Dexametazone chemotherapy and auto transplant. Pancytopenia, hyperproteinemia, severe inflammatory sindrome and tumoral lysis syndrome were noticed in the evolution of the hematological disease, though, with good clinical and paraclinical recovery process. He presented recently in our clinic with a very suggestive clinical profile of hyperthyroidism recurrence. Blood tests confirmed high level of thyroid hormones: TSH= 0.005 mU/l, fT4= 28.9 pmol/l (10.419.4), fT3= 7.9 pmol/l (2.46.8), TRAb= 4.7 U/l and normal full blood count. Despite the previous bone marrow suppression, in the absence of the possibility of radioiodine therapy, Methimazole treatment was started in a dosage of 10 mg per day with careful FBC monitoring. The evolution on low ATS dose was favorable and without adverse reactions till present. The transplant immunosuppression protocol can have favorable effect on the evolution of autoimmune diseases. In this case, a low ATS dose was sufficient to normalize the thyroid function. The association of multiple myeloma with autoimmune diseases is frequent, but the pathogenesis still remains unknown. Thyroid function must be performed periodically. ATS treatment should be administered only after taking into consideration the possible presence of pancytopenia.