Inhibition by thiocyanate of accumulation of radioiodine by thyroid gland

¹ Laboratory of Physiology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland

An investigation was made to determine whether the inhibition by thiocyanate of the thyroidal accumulation of radioiodine can be accounted for by the well-known inhibition of the iodide concentrating mechanism, or whether, in addition, there is a direct effect on the iodination process. At the lower concentrations of serum thiocyanate the decrease in the thyroidal accumulation of radioiodine was proportional to the ratio of the thyroid and serum radioiodide concentrations (T/S) and no appreciable effect on the rate of incorporation of radioiodide per unit of thyroidal radioiodide into protein-bound I¹³¹ (PBI¹³¹) was observed. At higher serum thiocyanate concentrations the fraction of the thyroid radioiodide incorporated into PBI¹³¹ per minute decreased as the uptake decreased whereas the T/S was relatively constant. However, at this higher range of concentrations no unambiguous interpretation of the effect of thiocyanate could be made because of the lack of information about the distribution of radioiodide in the internal compartments of the thyroid. A comparison was made also of the effect of thiocyanate on the T/S when the iodination process was permitted with the effect on the T/S when iodination was blocked.