A controlled prospective study was conducted to determine whether thyroid disorders are present with increased frequency in patients with MS. We found that thyroid disorders were at least three times more common in women with MS than in female controls. This was accounted for mainly by the prevalence of hypothyroidism among the female MS patients. Because hypothyroidism is usually due to Hashimoto's thyroiditis, its association with MS may support the hypothesis of autoimmune pathogenesis for MS. Our findings might have therapeutic implications because interferon treatment can induce antithyroid antibodies and thyroiditis.