Hypothyroidism, a condition in which the thyroid gland does not produce enough hormones, is not more likely to occur in people with one ANCA-associated vasculitis (AAV) subtype than the others, a study found. But the thyroid disease is associated with the presence of perinuclear ANCA (p-ANCA) antibodies or ANCAs against the myeloperoxidase protein (MPO-ANCA), particularly among patients with the granulomatosis with polyangiitis (GPA) subtype. The study with those findings, “Thyroid Disease in Patients with ANCA-Associated Vasculitis,” was presented at the American College of Rheumatology Convergence 2020, held virtually Nov. 5–9. AAV is a rare autoimmune disease defined by the inflammation and damage to small blood vessels. It can be classified into three subtypes: GPA, microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA). People with AAV, particularly those with MPO-ANCA, have been shown to have a greater prevalence of thyroid disease than the general population. However, the majority of patients in these studies had MPA, leaving it unclear whether this prevalence also is higher in people with other AAV subtypes. To address this gap in knowledge, a team of researchers in the U.S. and Canada evaluated the presence of thyroid disease among people with different AAV subtypes, and compared the clinical features of AAV in patients with and without thyroid disease. A total of 1,315 patients were included in the analysis, 55% of whom were female, and whose average age was 52.5 years. The most common subtype was GPA, affecting 866 (66%) patients, followed by EGPA (282 patients, 21%) and MPA (167 patients, 13%). In total, 145 of patients (11%) had hypothyroidism. Results showed that hypothyroidism was not associated significantly with any type of AAV, meaning this thyroid disease equally affected AAV patients regardless of their disease subtype. However, the presence of specific autoantibodies was associated with a greater likelihood of having hypothyroidism — 52% greater with p-ANCA antibodies and 89% greater with MPO-ANCA. While this association was observed in the overall population, the presence of p-ANCA and MPO-ANCA antibodies particularly increased the likelihood of hypothyroidism in GPA patients, researchers noted. In this subgroup of patients, having MPO-ANCA increased the odds of having hypothyroidism by threefold, compared with having ANCAs against the proteinase 3 protein. Meanwhile, having p-ANCAs increased those odds by 2.3 times, compared with having cytoplasmic ANCAs. The clinical features of AAV also were compared between patients with and without hypothyroidism. Overall, the two groups exhibited disease symptoms across the same organs, but those with hypothyroidism were 84% more likely to experience venous thromboembolism, which are clots that form inside a blood vessel. Although this study confirms the association of hypothyroidism with positive p-ANCA and MPO-ANCA in patients with AAV, researchers recognize the need for further investigation regarding the increased risk of venous thromboembolism and its mechanism. "The increased risk of venous thromboembolism in patients with AAV and hypothyroidism warrants further investigation and may be due to additional effects of hypothyroidism on [blood vessels] or [propensity for blood clots]," the researchers wrote.