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Methimazole‐induced agranulocytosis: Growth inhibition of myeloid progenitor cells by the patient's serum

The mechanism for agranulcytosis induced by antithyroid drugs is not established. The few available studies have proposed an immune‐mediated process against mature granulocytes. We investigated the effect of methimazole and propylthiouracil and serum from a patient with methimazole‐induced agranulocytosis on marrow myeloid colony growth. In the presence of normal serum or patient's recovery serum, antithyroid drugs had no effect on the growth of CFU‐GM colonies from normal or patient's marrow. However, the patient's serum obtained during agranulocytosis inhibited the in vitro myeloid colony growth from both autologous and allogeneic bone marrow. These results are compatible with an immune‐mediated mechanism for methimazole‐induced agranulocytosis rather than a direct toxic effect of the drug on abnormally sensitive cells.