Over the past 6 years, from May 1960 to June 1966, 1,716 patients with various diseases of thyroid were examined and thyroid function tests with $^{131}I$ were done. Among them, 545 patients with hyperthyroidism were treated with $^{131}I$. A summary of the clinical data of the $^{131}I$-thyroid function tests and the therapeutic results of $^{131}I$ were presented and discussed. 1. The patients examined consisted of; 596 cases(34.7%) with toxic diffuse goiter, 412 cases(24.0%) with non-toxic nodular goiter, 278 cases(16.2%) with euthyroidism, 236 cases(13.8%) with non-toxic diffuse goiter, 89 cases(5.2%) with hypothyroidism, 53 cases(3.1%) with toxic nodular goiter, 32 cases(1.9%) with thyroiditis and 20 cases(1.2%) with dyshormonogenesis. 2. There were 218(12.7%) male patients and 1,498(87.3%) female patients, showing a ratio of 1:6.9. female predominantly. 3. The majority of patients(79.6%) were in the 3rd through 5th decades of their lives showing the peak in the 4th decades(35.9%). 4. The diagnostic values and normal ranges of $^{131}I$ uptake test, 48 hour serum activity, $T_3$ red blood cell uptake and $PB^{131}I$ conversion ratio were discussed. 5. An attention was given to dyshormonogenesis, a qualitative hypothyroidism, due to its characteristic findings of clinical and $^{131}I$ thyroid function tests, and its pathogenesis was briefly reviewed. 6. Among 545 patients with hyperthyroidism treated with $^{131}I$, 68.3% was cured after single. therapeutic dose and another 24.0% was cured after second dose. 7. The complications of $^{131}I$ therapy were discussed in some details and myxedema had developed. in 3.9% of our cases. No thyroid cancer was found after $^{131}I$ therapy.