Urinary Iodine-to-Creatinine Ratio and Outcomes After Radioactive Iodine Ablation in Thyroid Cancer

Context Radioactive iodine ablation (RAIA) in differentiated thyroid cancer (DTC) requires elevated TSH levels, achieved through thyroid hormone withdrawal (THW) or recombinant human TSH (rhTSH). A low-iodine diet (LID) is recommended to enhance RAIA efficacy, but its impact on long-term outcomes remains uncertain. Objective To determine whether pre-ablation urinary iodine concentrations predict disease outcomes after RAIA. Study design Retrospective study of DTC patients treated with post-surgical RAIA at BIDMC from 2007-2020 Patients Of 463 patients reviewed, 304 met inclusion criteria (THW:189; rhTSH:115). All completed a 2-week LID. Urinary iodine-to-creatinine ratio (UICR) was measured as a marker of iodine status. Main Outcome Measure(s) Disease recurrence at 12 months, defined by elevated thyroglobulin (Tg), abnormal neck ultrasound, or a positive whole-body scan. Associations between UICR and outcomes were evaluated using univariable and multivariable logistic regression. Results Positive pre-treatment Tg (OR 6.13, 95% CI 1.90–19.83; p=0.002) and AJCC stage IV disease (OR 8.45, 95% CI 1.39–51.36; p=0.02) were independently associated with recurrence. Older age was associated with lower recurrence risk only in unadjusted models. UICR showed no association with outcomes. Although mean UICR was higher in the rhTSH group (p<0.0001), recurrence rates were similar between THW and rhTSH (p=0.3). Conclusions Higher UICR in rhTSH-prepared patients did not translate into worse outcomes. Pre-treatment Tg and stage IV disease were the strongest predictors of recurrence, whereas iodine status was not. These findings suggest that strict iodine depletion at the level achieved here, reflected by UICR, may not determine RAIA efficacy in DTC.