Prevalence of positive anti-neutrophil cytoplasmic antibody (ANCA) in patients receiving anti-thyroid medication.

OBJECTIVE Vasculitis is a rare complication of anti-thyroid medications. There are 32 cases of anti-neutrophil 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 +/- s.d. of 7.9+/-10.2 years, compared with 0.8+/-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 ANCA-positive 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.