A Novel Protocol for Pre-Treatment Diagnostic 131 I Scan Within 24 Hours Post-Second rhTSH Administration to Guide Radioiodine Therapy in Differentiated Thyroid Cancer

Background: In the current conventional recombinant human thyrotropin (rhTSH) protocol, radioiodine therapy (RAIT) is given at 24 h post-second injection and diagnostic 131 I whole-body scan (Dx-WBS) at 72 h; hence, Dx-WBS cannot precede RAIT within the same two-injection cycle. This study aimed to evaluate whether Dx-WBS could be performed within 24 hours after the second injection of a novel rhTSH, thereby enabling timely pre-RAIT evaluation without altering the established rhTSH-based RAIT schedule. Methods: This single-center, prospective trial enrolled patients with differentiated thyroid carcinoma (DTC) who had undergone total thyroidectomy. All patients received rhTSH (0.9 mg daily) on 2 consecutive days. On the same day, about 6 hours after the second rhTSH injection, a low-dose diagnostic activity of 131 I was given, followed by Dx-WBS at approximately 12, 18, and 24 hours post-administration. RAIT was administered on the same day following the 24-hour Dx-WBS, and post-treatment whole-body scan (Rx-WBS) served as the reference standard for assessing concordance with Dx-WBS. Thyroglobulin (Tg) and anti-thyroglobulin antibody (TgAb) levels were measured at baseline and at 6, 24, and 72 hours following the second rhTSH injection. In TgAb-negative patients, sTg values were stratified into three categories (<1, 1–10, and ≥10 ng/mL), and concordance of 6h- and 24h-sTg with 72h-sTg was assessed based on identical categorization. Results: A total of 110 patients were eventually enrolled, concordance rates of 12h-, 18h-, and 24h-Dx-WBS with Rx-WBS were 82.7% [CI: 74.3–89.3%], 91.8% [CI: 85.0–96.2%], and 92.7% [CI: 86.2–96.8%], respectively. Concordance at both 18 and 24 hours was significantly higher than that at 12 hours ( p < 0.05). The concordance rate of 24-hour sTg with 72-hour sTg was significantly higher than that of 6-hour sTg (89.3%, [CI: 80.1–95.3%] vs. 72.0%, [CI: 60.4–81.8%], p < 0.05). Conclusions: The rhTSH-aided Dx-WBS and sTg, performed within 24 hours post-second rhTSH injection, are reliable and efficient in enabling pre-RAIT disease evaluation and subsequent RAIT within the same two-injection cycle, providing a cost-effective strategy.