Context: The clinical characteristics of myeloperoxidase antineutrophil cytoplasmic antibody (MPO-ANCA)-associated vasculitis caused by antithyroid drugs are still unclear because most reports describe only a small number of patients.

Objective: The objective was to analyse a large number of patients with MPO-ANCA-associated vasculitis to determine the time of onset, the drug and dose taken, the clinical symptoms, the relationship between the clinical symptoms and the MPO-ANCA titre, and the incidence.

Design: We analysed 92 patients in whom the adverse reaction of MPO-ANCA-associated vasculitis was reported to Chugai Pharmaceutical, a company that markets antithyroid drugs.

Results: Of the 92 patients, 41 (44.6%) had single-organ failure, 32 (34.8%) had two-organ failure, 13 (14.1%) had three-organ failure and two (2.2%) had four-organ failure. The number of organs involved was unknown in the other four patients (4.3%). The median time of onset was 42 months (range: 1 to 372 months) after starting drug treatment. The median dose at onset of MPO-ANCA-associated vasculitis was 15 mg/day (range: 2.5 to 45 mg/day) for methimazole and 200 mg/day (50 to 450 mg/day) for propylthiouracil. The severity and number of organs involved were not correlated with the MPO-ANCA titre. The incidence was between 0.53 and 0.79 patients per 10,000 and the ratio of the estimated incidences for methimazole and propylthiouracil was 1:39.2.

Conclusions: The time of onset of MPO-ANCA-associated vasculitis and the dose at onset varied. The severity and number of organs involved were not correlated with the MPO-ANCA titre, indicating a need for vigilance even when the MPO-ANCA titre is only weakly positive.