The T3 suppression test is based on the principle that there is a negative feedback mechanism between the hypothalamus-pituitary and thyroid axes. The T3 suppression test is used to diagnose atypical hyperthyroidism or T3-type hyperthyroidism, and can also be used as an indicator of relapse after discontinuation of long-term antithyroid medication. For patients with hyperthyroidism and simple goiter, the uptake rate of iodine-131 is not inhibited after oral administration of T3 in patients with hyperthyroidism, but the uptake rate of iodine-131 in patients with simple goiter is significantly inhibited after oral administration of T3 through the pituitary’s negative feedback mechanism.