Objective: Vasculitis is a rare complication of anti-thyroid medications. There are 32 cases of antineutrophil cytoplasmic antibody (ANCA)-positive vasculitis in association with anti-thyroid medication reported in the English literature. The objectives of this study were to assess the frequency of positive ANCA in patients on long-term anti-thyroid medication, and to follow patients prospectively from commencement of medication to determine whether they became ANCA-positive after therapy. Design: Prospectively collected cross-sectional study of two groups of patients: (i) who had received long-term (>18 months) anti-thyroid medication, and (ii) newly diagnosed thyrotoxicosis before commencement of anti-thyroid medication attending clinic between 28 April 1998 and 30 September 1998. Data were collected for age, sex, ethnicity, underlying thyroid disease, medication and duration, and symptomatology. Results: Eight of 30 patients on long-term anti-thyroid medication (26.7%) were ANCA-positive. All ANCA-positive patients were female, seven were taking propylthiouracil (PTU) at the time of testing. ANCA-positive patients had taken PTU for a mean Ϯ S.D. of 7.9 Ϯ 10.2 years, compared with 0.8 Ϯ 2.2 years in ANCA-negative patients (Mann-Whitney, P < 0.0001). The ten patients with newly diagnosed thyrotoxicosis were ANCA-negative before commencement of carbimazole. One (10%) became ANCApositive within 8 months of therapy. Conclusions: In our population, ANCA-positivity in association with long-term anti-thyroid medication is common (26.7%). One patient who was ANCA-negative prior to anti-thyroid therapy has become ANCA-positive. ANCA should be tested in patients receiving long-term anti-thyroid medications, and in patients with adverse reactions. As PTU is more commonly associated with vasculitis and positive ANCA, carbimazole may be the preferred medication for long-term use. Patients with positive ANCA should be followed, and considered for definitive anti-thyroid therapy, to allow cessation of medication. ANCA-positivity may resolve after cessation of anti-thyroid medication.

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