The authors studied on the 5 cases of solitary thyroid nodule by T3 suppression test and TSH stimulation test. Radioiodine uptake and thyroid scan were observed after administration of dssicated thyroid and TSH. 3 of 5 cases were teated by $^{131}I$ and 2 by long-term adiministration of dessicated thyroid. Following were the results: 1. Nodular tissue was not affected by the administration of dessicated thyroid or TSH. 2. Extranodular tissue responded as normal thyroid tissue on the administration of dessicated thyroid or TSH. 3. There were many gradations from euthyroid to hyperthroid in clinical state. 4. Treatments were succesful in all cases except one case who was treated by long-term administration of dessicated thyroid.