Risk Factors for Indeterminate Response After Radioactive Iodine Therapy in Patients With Differentiated Thyroid Cancer Purpose We investigated the risk factors of indeterminate response (IDR) in patients who underwent recombinant human thyroid-stimulating hormone (rhTSH)–aided radioactive iodine therapy (RAIT). Methods A total of 128 patients with papillary thyroid cancer were included in this retrospective study. The patients were classified into excellent response and IDR groups based on follow-up diagnostic whole-body scintigraphy (WBS) and TSH-stimulated thyroglobulin (Tg). Indeterminate response was defined as the presence of a faint uptake in the thyroid bed on the diagnostic WBS or a TSH-stimulated Tg detectable, but less than 10 ng/mL. Parameters that act as significant risk factors for IDR, including age, sex, stage, surgeon, time interval between surgery and RAIT, post-treatment WBS finding, urine iodine-to-creatinine ratio, TSH-unstimulated Tg, and rhTSH-stimulated Tg, were analyzed using a Cox proportional hazards regression method. Results After treatment, 64 patients showed IDR. Recombinant human TSH–stimulated Tg was the only independent risk factor for predicting IDR. Patients with an rhTSH-stimulated Tg greater than 2 ng/mL prior to RAIT were 3.75 times more likely (95% confidence interval, 1.61–8.72) to have an IDR than those with a lower rhTSH-stimulated Tg (≤2 ng/mL). Conclusions Pre-RAIT TSH-stimulated Tg levels are a risk factor for IDR after RAIT.