To attain a simple and reliable method of evaluating the thyroid function the reliability of the clinical manifestation and the conventional thyroid function tests in diagnosing the hyperthyroidism was studied. The subjects included 184 patients with hyperthyroidism and 66 cases with euthyroidism, who were treated at the Thyroid Clinic, Seoul National University Hospital, from July 1971 through August 1972. The observed results were as follows: 1. In the cases of hyperthyroidism, 19% of the patients were male and 81% female; in the cases of euthyroidism, 7.6% of the patients were male and 92.4% female. The majority of the patients were in 2nd to 4th decades of their lives. 2. There were objective signs clearly manifested in hyperthyroidism which were rare or absent in the euthyroid state. These clinical signs included wide pulse pressure, tachycardia, systolic murmur, exophthalmos, tremor, and warm skin. In the hyperthyroid state 91.3% of the cases manifested two or more of the above signs, whereas in the euthyroid state no patients manifested any two of the above signs. 3. The most frequent complaints of the patients with thyroid disease were palpitation, weight loss, increased appetite, heat intolerance, perspiration, hunger feeling; nervousness, exertional dyspnea, etc. There was no clear difference in the incidence of the symptoms between hyperthyroidism and euthyroidism. 4. In the diagnosis of hyperthyroidism, the reliability of thyroid ?unction tests was as follows; $T_7$ was 92.4% reliable, $^{125}IT_3$ resin uptake rate 91.6% reliable, $^{131}I$ thyroid uptake rate in 24 hrs. 89.4% reliable, serum $T_4$ level 85.9% reliable and BMR 75.5% reliable. Therefore the careful observation of the clinical manifestation of the disease is a simple and reliable way of making a correct diagnosis of either hyperthyroidism or euthyroidism. 5. In hyperthyroidism there shows no correlationship between the results of the thyroid function test and clinical signs but a high BMR was associated with both tachycardia and systolic murmur.