A clinical study was made on 263 patients of hypothyroidism among the 5,970 patients of Various thyroid diseases diagnosed and treated at the Radioisotope Clinic and Laboratory, Seoul National University Hospital from May, 1960 to Aug, 1975. The results obtained with this study are as follows: 1) The etiological classification of hypothyroidism reveaied 244 cases (93%) of primary hypothyroidism and 19 cases (7%) of secondary hypothyroidism. 2) The most frequent cause of the thyroprivic primary hypothyroidism was post radioiodine therapy with 109 cases (41.4%). 3) There were 37 cases (14%) of male and 226 cases (86%) of female, showing a ratio of 1:6. 4) The majority of patients were between the ages of 30 and 60 with the peak incidence (87 cases, 33%) in their fourth decades of lives. 5) The major symtoms and signs were weakness (97%), edema of face and extremities (92%); Decreased Achilles tendon reflex (87%), cold intolerance (82%), gain in weight (76%), constipation (58%) and cold skin (51%). 6) The cumulative incidence of hypothyroidism in patient treated with 131I (3∼8mci) was 7.2% at first year, 33.3% at tenth year and the 50% at fourteenth year and the annual increment was 2.9%. 7) The incidence of hypothyroidism related to thenumbers of 131I therapy was not linear. 8) The diagnostic compatibilities of the various tests to hypothyroidism were TSH (100%), T4 (93.8%), 24-hour-RNIU (91.5%), ATR (86.7%), T3RU (66.1%) and BMR (64.9%).