Dynamic Thyroglobulin Ratio as a Biomarker to Identify Papillary Thyroid Cancer Patients Who Would Benefit from a Low-Iodine Diet

<b>Objectives</b>: This study aimed to assess whether low-iodine diet (LID) adherence is associated with therapeutic response in papillary thyroid carcinoma (PTC), specifically in relation to post-therapeutic thyroglobulin (Tg) release as a surrogate marker for the acute radiation-induced response following radioactive iodine (RAI) therapy. <b>Methods</b>: This retrospective study included 895 patients with PTC treated with RAI. LID adherence was assessed using the urine iodine-to-creatinine (I/Cr) ratio, with <66.2 μg/g Cr defined as good adherence. The Tg ratio (ratioTg), calculated by dividing post-RAI Tg (measured 7 days after RAI) by pre-RAI Tg, was used to reflect the magnitude of the radiation-induced Tg release. Patients were stratified by ratioTg (≤1 vs. >1), and associations between LID adherence and therapeutic response were analyzed within each group. <b>Results</b>: Well-adherent patients exhibited significantly higher ratioTg compared to poorly adherent patients (15.7 ± 2.2 vs. 8.9 ± 1.3, <i>p</i> = 0.007). Among patients with ratioTg > 1 (<i>n</i> = 630), LID adherence was independently associated with improved therapeutic response (OR, 2.004; 95% CI, 1.270-3.162; <i>p</i> = 0.003). No such association was observed in patients with ratioTg ≤ 1 (<i>n</i> = 265; <i>p</i> = 0.546). <b>Conclusions</b>: The clinical benefit of LID appears to depend on the presence of a certain magnitude of radiation-induced Tg release. RatioTg may serve as a useful marker for identifying patients likely to benefit from LID.