Suppression of Thyroidal Iodine Uptake After Indocyanine Green

ABSTRACT The 24-hr thyroidal uptake of radioiodine (RAIU) is suppressed following administration of indocyanine green, a dye commonly used to measure cardiac output. Studies were conducted in 21 normal subjects and 6 patients with hyperthyroidism to determine the mechanism responsible for this suppression and to evaluate its clinical significance. Analysis of the dye revealed that each 25 mg dose contained 1.274 ± 0.68 mg of inorganic iodide. Administration of indocyanine green had no effect on the protein-bound iodide concentration but significantly increased the serum inorganic iodide concentration of both groups. The most striking effect of indocyanine green was suppression of the RAIU. In normal subjects the mean decrease in RAIU was 6% but only 8 of the 21 subjects had a decrease of 5% or greater. In contrast, the mean decrease in RAIU of patients with hyperthyroidism was 33% and 4 of the 6 patients had normal values for RAIU after administration of the dye.