To diagnosis and understand the pathophysiologic status of thyroid disease, not only hormonal measurments but also thyroid scan is believed to have a unique role. Especially in the cases of the change of the thyroid function by thyroiditis, it is emphasized that thyroid scan can be helpful in differentia! diagnosis, Discordant results of thyroid hormone levels and thyroid scan are found in transient hyperthyroidism, or in transient hypothyroidism. We analysed and reevaluated thyroid scan to look at the importance of thyroid scan. The results are summerised as follows: 1) 80%. of hyperthyroid patients had hyperthyroidism increased RAIU with even density, they are compatible with Graves' disease. 2) 2.1% of hyperthyroid patients had normal or decreased RAIU, which are classfied as high iodine turn over genuine hyperthyroidism. 3) 8.5% of hyperthyroid patients had markedly decreased RAIU at both 2 hour and 24 hour, whose patholgic processes are suggested to be heterogenous namely subacute thyroiditis, postpartum thyroiditis, Hashimotos thyroiditis, and pamless thyroiditis. 4) 45% of hypothyroid patients had increased 24hr RAIU, 30% of hypothyroid patients were normal, 25%, decreased In conclusion. thyroid scan should be reevaluated its useful role to asses the pathophysiologic status of thyroid disease. Especially in cases of transient thyrotoxicosis, thyroid scan is essential to diagnose and follow up the disease process.