For the assessment of antithyroid effect of lithium carbonate, it was administered to the 17 hyperthyroid and 5 euthyroid patients, who visited the Seoul National University Hospital from Jan. to Aug., 1977. Thyroid function tests were performed just before the admini stration of Lithium carbonate, 2 weeks and 2 months after lithium treatment. The results were as follows; 1) In the 5 euthyroid patients, no significant changes in thyroid function tests were obtained before and after lithium treatment. 2) In the 17 hyperthyroid patients, the values of the T3RIA were 370±121ng/dl 2 weeks after lithium treatment as compared with 506±12lng/dl before the administration, of which the mean percentage fall was 26.9%. T3RU was varied from 56.8±8.0% to 47.3±8.1% (16.7% in mean pcentage fall), T4 was changed from 24.2±2.4ug/dl to 22.0±4.2ug/dl (9.1% in meanfall), and T7, from 13.82±2.25 to 10.55±3.12 (23.7% in mean fall). 3) In the 5 hyperthyroid patients, serial thyroid function tests were performed 2 weeks and 2 months later. The mean percentage falls of T3RIA were 36.6 and 61.3%, 2 weeks and 2 months after lithium treatment respectively. Those of T3RU were 17.5 and 35.1%, those of T4 were 20.4 and 44.0%, T7, 35.0 and 60.7%. 4) Approximately 45∼60% of mean fall in thyroid function tests were obtained within the second week. Normal thyroid function tests were observed in 2 among 17 patients within the second week, and 2 among 5 patients within the second month. 18 patients, however, became clinically euthyroid within the 4th week. 5) Single case of hypothyroidism was experienced, and 5 patients (29.4%) complained mild side effect. Lithium salts could be safely administered to hyperthyroid patients who are allergic to thioamides or iodine. Its use is indicated in cases of acute thyrotoxicosis in which it's necessary to reduce hormone levels very rapidly, and lithium-thioamides drug combination is a highly effective and safe means of initial routine control of hyperthyroidism.