Association of post-therapy serum 131I counts with treatment outcomes in differentiated thyroid cancer: a prospective observational study

We investigated the association of post-therapy serum 131 I counts with treatment outcomes in differentiated thyroid cancer. We prospectively enrolled patients receiving high-activity 131 I as initial postoperative radioactive iodine (RAI) therapy (n = 84), repeat therapy (n = 24), or metastatic disease treatment (n = 23). Serum 131 I counts and day-7 whole-body scintigraphy (WBS) were evaluated against treatment success and recurrence in the non-metastatic cohort, and favorable response (≥ 25% TSH-suppressed thyroglobulin [on-Tg] reduction) and progressive disease in the metastatic cohort. In non-metastatic patients, serum 131 I counts strongly correlated with WBS neck counts (ρ = 0.778, p < 0.001), whereas stimulated thyroglobulin (off-Tg) correlated with WBS counts only in treatment-naïve patients (ρ = 0.334, p = 0.001). Treatment success was independently associated with high serum 131 I counts (OR 11.02) and low off-Tg (OR 88.32), and serum 131 I counts further stratified outcomes within off-Tg-defined groups. In metastatic patients, serum 131 I counts were more strongly associated with on-Tg reduction than off-Tg (OR 45.50 vs. 19.27), and low serum 131 I counts were associated with progressive disease. Findings in the metastatic cohort should be considered exploratory given the small sample size and limited number of progression events. Post-therapy serum 131 I counts correlated with WBS uptake and were associated with subsequent treatment outcomes.