RELATIVE MEASUREMENT IN VIVO OF ACCUMULATION OF RADIOIODINE BY THE HUMAN THYROID GLAND: COMPARISON WITH RADIOACTIVITY IN PERIPHERAL TISSUES 1

Abstract

A method is described for making relative in vivo measurements of the quantity of radioiodine accumulated by the thyroid. Accumulation of radioiodine by the thyroid was measured in 29 individuals having various states of thyroidal function. These measurements were compared with observations over the extremities (thighs), with measurements of urinary excretion and with analysis of blood for radioiodine. In vivo observations over the thyroid show substantially more radioiodine accumulated by the thyroids of hyperthyroid individuals than by the thyroids of euthyroid individuals. Euthyroid patients with nodular goiters may at times collect more radioiodine in their thyroids than other euthyroid subjects. Hypothyroid patients show little or no accumulation of iodine in their thyroids. The accumulation of radioiodine by the thyroid was observed to follow an exponential curve similar in form to the curve of urinary excretion of radioiodine. Observations of radioactivity over the thigh, on the other hand, closely resembled curves constructed from analyses of radioiodine in serum. Estimation of the exponential rate constant of the curve of radioiodine accumulation by the thyroid was found to have the same value as the rate constant estimated from observations over the thigh, from observations of urinary excretion or from analyses of blood for radioiodine. This rate is regarded as representing the rate of disappearance of radioiodine from blood no matter which method is used to obtain it. The rate of disappearance of radioiodine from blood varies according to the state of thyroidal activity. Even when obtained directly from observations on the thyroid in vivo, however, it does not provide a direct or quantitative measure of the accumulation of radioiodine by the thyroid, since it must measure the rate of disappearance of radioiodine from the blood into all sites of disposal, including the urine and other tissues as well as the thyroid.