The function of thyroid gland is influenced by complex factors. In order to clarify the mechanism of increaseduptake rate of radioactive iodine I131 by the toxic thyroid gland, the authors calculated the 131I uptake rate ofeach gram of thyroid tissue of toxic and non-toxic patients and it was found that the relative 131I uptake rate ofthyroid gland in the thyrotoxic patients was slightly higher than that of non-toxic cases, and it was concludedthat the incraesed 131I uptake rate in toxic thyroid glands was mostly due to increased size and weight of theglands and partially due to increased iodine uptake rate of thyroid tissue itself.