The effects of kojic acid (KA) on thyroidal function were studied by single-dose administration in rats and in cultured rat thyroid cells (FRTL-5 cells). In rats receiving a single dose of 1,000 mg/kg KA orally, the ¹²⁵I uptake from blood into the thyroid gland was significantly lower than that of the control group from 30 min to 24 hr after administration. The ¹²⁵I organification activity of the KA groups was significantly lower than control from 30 min to 6 hr after administration. However, the ¹²⁵I organification activity at 24 hr or 48 hr after administration recovered enough to be nearly comparable with the control group. In the study in FRTL-5 cells, KA inhibited iodine organification dose-dependently, but did not inhibit iodine uptake. These results suggest that the observed lower iodine uptake activity in the single-dose administration study in rats was due to the inhibition of iodine organification caused by the oral administration of KA, consequently decreasing iodine in the entire thyroid gland. Although serum T4 showed a tendency to decrease from 2 hr to 48 hr after administration of KA, serum TSH did not show any evident change associated with KA in the single-dose administration study in rats. Based on these results, it is presumed that a massive dose or long administration period might be needed to decrease serum T4 and increase serum TSH. From these results, it is presumed that KA affected thyroidal function when given at a massive dose or in a long administration period by inhibiting iodine organification in the thyroid.