Iodine-131 uptake and turnover rate vary over short intervals in Graves' disease

From a Dutch questionnaire, it was apparent that nearly all institutions used percentage of radioiodine uptake for calculation of the radioiodine dose in Graves' disease. Although there is a general belief that fluctuations in radioiodine uptake may occur, with few exceptions relatively long intervals were accepted between the uptake measurement and the actual therapy dose. With the aim of optimizing the pretherapeutic work-up, we evaluated the stability of iodine uptake over time in patients with Graves' disease who were referred for 131I therapy. 131I uptake was measured in 300 consecutive patients for the calculation of the required 131I therapy dose; data were complete for 291 patients (97%). After discontinuing thyroid medication for 3 days, standardized thyroid probe measurements were performed 5 and 24 h after ingestion of a capsule containing 0.37 MBq 131I-NaI. Measurements were performed at the time of scintigraphic diagnosis (test 1), as well as immediately before 131I therapy (test 2). The time interval between test 1 and test 2 ranged from 2 to 421 (median 40) days. A relative increase or decrease greater than 10% between tests 1 and 2 occurred in 180 of 291 cases (62%) at 5 h and in 158 of 291 patients (54%) at 24 h. These changes were not related to the interval between the tests or to initial uptake values, thyroid mass, gender or age. Rapid turnover of radioiodine (5 h/24 h uptake ratio > 1) was noted in 17% of the patients during test 1 and in 15% during test 2. Rapid turnover was persistent (present in both tests 1 and 2) in only 9%. We conclude that patients with Graves' disease show considerable changes in 131I uptake over relatively short periods of time, and the turnover rate of 131I in this condition is not constant.