Thioureas agranulocytosis with

Antithyroid drugs for long-term therapy (C). Thiourea-derivatives (thioamides) inhibit peroxidase and, hence, hormone synthesis. To restore a euthyroid state, two therapeutic principles can be applied in Graves disease (a) monotherapy with a thioamide, with gradual dose reduction as the disease abates (b) administration of high doses of a thioamide, with concurrent administration of thyroxine to offset diminished hormone synthesis. Adverse effects of thioamides are rare, but the possibility of agranulocytosis has to be kept in mind. 

Perhaps one of the most common side effects is a benign transient leukopenia characterized by a white blood cell (WBC) count of less than 4000/mm. This condition occurs in up to 12% of adults and 25% of children, and sometimes can be confused with mild leukopenia seen in Graves disease. This mild leukopenia is not a harbinger of the more serious adverse effect of agranulocytosis, so therapy can usually be continued. If a minor adverse reaction occurs with one antithyroid drug, the alternate thiourea may be tried, but cross-sensitivity occurs in about 50% of patients. ... 

---