Alopecia areata (AA) has long been associated with thyroid diseases; however, the temporality of their association remains unclear. This study aimed to investigate the bidirectional association between AA and thyroid diseases. In analysis 1, we included 5929 AA patients and 59,290 matched controls to assess the risk of thyroid diseases. In analysis 2, we included 35,071 patients with thyrotoxicosis, 19,227 patients with Graves’ disease, 5460 patients with thyroiditis, 3352 patients with Hashimoto’s thyroiditis, and their matched controls (1:10) to assess the risk of AA. Incidence of thyroid diseases and AA were the outcomes in analysis 1 and analysis 2, respectively. After adjusting the potential confounders, AA patients had an increased risk of all thyroid diseases, including toxic nodular goiter, (aHR 10.17; 95% confidence interval [CI] 5.32–19.44), nontoxic nodular goiter (aHR 5.23; 95% CI 3.76–7.28), thyrotoxicosis (aHR 7.96; 95% CI 6.01–10.54), Graves’ disease (aHR 8.36; 95% CI 5.66–12.35), thyroiditis (aHR 4.04; 95% CI 2.12–7.73), and Hashimoto thyroiditis (aHR 4.35; 95% CI 1.88–10.04). On the contrary, a significantly increased risk of developing AA was observed among patients with thyrotoxicosis (aHR 9.29; 95% CI, 7.11–12.14), Graves’ disease (aHR 8.66; 95% CI 6.03–12.42), and thyroiditis (aHR 6.42; 95% CI 3.15–13.11) but not in patients with Hashimoto’s thyroiditis. In conclusion, our study found a bidirectional association between AA and thyroid diseases, suggesting shared biological mechanisms underlying these two diseases.
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