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 thyroid-stimulating hormone (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. Methods: This retrospective study evaluated DTC patients treated with post-surgical RAIA at BIDMC from 2007 to 2020. Of 463 patients reviewed, 304 met inclusion criteria (THW: 189; rhTSH: 115), all completing a 2-week LID; urinary iodine-to-creatinine ratio (UICR) was measured as iodine status marker. Disease recurrence at 12 months, defined by elevated thyroglobulin (Tg), abnormal neck ultrasound, or a positive whole-body scan, was the primary outcome, with associations between UICR and outcomes evaluated using univariable and multivariable logistic regression. Results: Positive pre-treatment Tg (odds ratio [OR] 6.13, 95% CI 1.90-19.83; Conclusion: 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.