Four- to twenty-four-hour uptake ratio: an index of rapid iodine-131 turnover in hyperthyroidism

Rapid thyroidal iodine turnover may contribute to 131I therapy failure in patients with hyperthyroidism. The utility of a 4- to 24-hr 131I uptake ratio was evaluated as an index of thyroidal iodide retention in hyperthyroid patients.

In 433 hyperthyroid patients, the success of 131I therapy was correlated with the following factors: gender, pretreatment with antithyroid drugs, clinical diagnosis, magnitude of early and late thyroidal 131I uptake values, and the 4- to 24-hr 131I uptake ratio.

Of the 433 patients, 362 patients (84%) had a successful outcome after a single therapeutic dose of 131I while 71 (16%) did not. Multiple linear regression analysis revealed that the highest statistically significant predictor of outcome was the 4- to 24-hr 131I uptake ratio (p-value < 0.001); all other factors showed a weaker association. An 131I uptake ratio of > 1 was found in 67 (15%) patients. Thirty-two of these 67 patients (48%) failed 131I therapy, whereas those patients with uptake ratios of < 1.0, only 39/366 (11%) failed 131I therapy.

The 4- to 24-hr 131I thyroidal uptake ratio is a practical substitute for exact determination of the effective half-life. It identifies patients who are likely to have a rapid 131I turnover without the need for extended thyroid uptake measurements. An 131I uptake ratio of > or = 1 was found in 15% of hyperthyroid patients and was associated with a near 50% 131I therapy failure rate.