Thyroid function tests including thyroxine (T4), T3 test, thyroid-stimulating hormone (TSH), free T4 index and antithyroid antibodies were studied in 51 patients before the start of long-term lithium therapy and after 4 and 12 months on lithium. After 4 months on lithium, changes occurred in all laboratory parameters - T4 and T3 test levels decreased and TSH increased. After 12 months T4 and T3 test increased to pretreatment levels and TSH remained higher than before the start of lithium. During 1 year of lithium treatment only 1 patient developed clinical signs of hypothyroidism and started levo-thyroxine substitution therapy.