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

Objective: Vasculitis is a rare complication of anti-thyroid medications. T here are 32 cases of antineutrophil cytoplasmic antibody (ANCA)-positive va sculitis in association with anti-thyroid medication reported in the Englis h literature. The objectives of this study were to assess the frequency of positive ANCA in patients on long-term anti-thyroid medication, and to foll ow patients prospectively from commencement of medication to determine whet her they became ANCA-positive after therapy. Design: Prospectively collected cross-sectional study of two groups of pati ents: (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. D ata were collected for age, sex, ethnicity, underlying thyroid disease, med ication 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 taki ng propylthiouracil (PTU) at the time of testing. ANCA-positive patients ha d 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 p atients with newly diagnosed thyrotoxicosis were ANCA-negative before comme ncement of carbimazole. One (10%) became ANCA-positive within 8 months of t herapy. Conclusions: In our population, ANCA-positivity in association with long-te rm anti-thyroid medication is common (26.7%). One patient who was ANCA-nega tive prior to anti-thyroid therapy has become ANCA-positive. ANCA should be tested in patients receiving long-term anti-thyroid medications, and in pa tients with adverse reactions. As PTU is more commonly associated with vasc ulitis and positive ANCA, carbimazole may be the preferred medication for l ong-term use. Patients with positive ANCA should be followed, and considere d for definitive anti-thyroid therapy, to allow cessation of medication. AN CA-positivity may resolve after cessation of anti-thyroid medication.