The function of thyroid gland is influenced by complex factors. In order to clarify the mechanism of increased uptake rate of radioactive iodine Isup 131 by the toxic thyroid gland, the authors calculated the sup 131I uptake rate of each gram of thyroid tissue of toxic and non-toxic patients and it was found that the relative sup 131I uptake rate of thyroid gland in the thyrotoxic patients was slightly ghidher than that of non=toxic cases, and it was concluded that the increased sup 131I uptake rate in toxic thyroid glands was mostly due to increased size and weight of the glands and partially due to increased iodine uptake rate of thyroid tissue itself.