This paper reviews the literature and own data on prevalence of thyroid diseases in patients with multiple sclerosis (MS). It has been shown that 20-25% of untreated MS patients have autoimmune thyroiditis (AIT) and/or subclinical hypothyroidism. No correlation was found between age, gender, disease course, MS duration, disability status and prevalence of thyroid disorders. Disease-modifying therapy (DMT) of MS can induce or worsen the thyroid pathology. Female gender, pre-existing AIT, family history of thyroid disorders are considered as risk factors of thyroid dysfunction and autoimmunity during interferon- (IFN- ) therapy. Most cases of autoimmune hyperthyroidism require discontinuation of IFN- therapy. The present study of thyroid status includes 43 MS patients, 42% of them with thyroid disorders such as thyroid nodes and subclinical hypothyroidism. Thyroid dysfunction is revealed mostly in patients treated with IFN- -1b; thyroid notes - in MS patients receiving copaxone. These facts emphasize the significance of systematic thyroid assessment in patients with MS especially during the long-term DMT.