Uninhibited Thyroidal Uptake of Radioiodine Despite Iodine Excess in Amiodarone-Induced Hypothyroidism*

Iodine excess is associated with a low thyroidalradioiodine uptake due to dilution of the radioisotope by theincreased stable iodide pool. We studied thyroidal uptake ofradioisotopes in cardiac patients with iodine excess due to amiodarone treatment. 99mTc-pertechnetate scintigraphy was performedin 13 patients receiving long term amiodarone therapy.Five patients had a clearly visible thyroid gland, and 8 patientshad no or a very faint thyroid image. All patients with positivescans had an increased plasma TSH level, whereas all patientswith negative scans had a normal or absent TSH response toTRH. Thyroidal uptake and discharge of 123I were studied in 30other patients. Group I (n = 11) had normal plasma TSHresponses to TRH and no iodine excess, group II (n = 7) hadnormal TSH responses to TRH and excess iodine from metrizoateangiography in the previous month, group III (n = 7) hadnormal or decreased TSH responses to TRH while receivinglong term amiodarone therapy, and group IV (n = 5) hadincreased TSH respo...