One hundred Thai patients with chronic idiopathic urticaria (CIU) were studied to determine the prevalence of thyroid antibodies in comparison to 100 age- and sex-matched healthy volunteers. It was found that 21 patients (21%) with CIU were positive for thyroid antibodies. Among normal volunteers, only nine cases (9%) had elevated titers of thyroid antibodies. The frequency of thyroid antibodies in the chronic urticaria group was significantly more common than the control group (chi-square = 4.75, p=0.03). Among 21 patients with CIU who were positive for thyroid antibodies, 9 cases were negative for thyroid antibodies when repeating the tests after 3 months. Only 12 patients still had persistent elevation of antibodies after 3 months and thyroid function test was performed in these 12 patients. It was found that 9 cases had autoimmune thyroiditis with euthyroidism. One case had subclinical hyperthyroidism. One case had autoimmune hyperthyroidism, One case had subclinical hypothyroidism.