The relationship between 24 h/4 h radioiodine-131 uptake ratio and outcome after radioiodine therapy in 1402 patients with solitary autonomously functioning thyroid nodules.

To evaluate the role of 24 h/4 h uptake ratio (UR) in response to radioiodine-131 (131I) therapy in patients with autonomously functioning thyroid nodules (AFTN). A total of 1402 consecutive hyperthyroid patients were treated with 131I, between 1958 and 2005. Therapeutic doses (D) were calculated according to the formula: D = weight of nodule 9 dose per gram of nodular tissue (q)/24 h 131I uptake. The ratios of the 24 and 4 h uptake were retrospectively calculated and the patients were grouped according to outcome and q into three groups of UR (B1.25; 1.26–1.68; C1.69) by means of terziles. Of the 1402 patients, 95 did not respond to 131I treatment while 93/1307 developed hypothyroidism. Most non-responders (55.8%) had UR B1.25, while many hypothyroid patients (66.7%) had UR C1.69 (v2: P\0.001). As q increased, the proportion of successfully treated patients increased (level of significance) only in the group with UR B1.25; while in the other two terziles, with increasing dose per gram of nodular tissue, the number of successfully treated patients did not increase (level of significance). The cumulative incidence of hypothyroidism was 2.2% at the 1st year after 131I treatment, increasing to 13.9% at 5 years and 26.2% at 10 years. The 131I UR can predict the outcome of 131I treatment in AFTN and may have utility in modifying treatment in some patients to limit post-radioiodine induced hypothyroidism and treatment failures in order to achieve euthyroidism.