Prolonged Normal Thyroid Function After 131I Radioiodine Therapy Using a Minute LT3 Suppression Test (LT3s-RIT) in Patients with Thyroid Unifocal Autonomy and Baseline Detectable TSH

<b>Background</b>: Subclinical hyperthyroidism grade 1 (SCH G1, TSH > 0.1 mU/L) is common in patients with thyroid unifocal autonomy (UFA) and associated with cardiovascular risks and increased mortality. While ¹³¹I radioiodine therapy (¹³¹I-RIT) effectively treats UFA, it frequently induces hypothyroidism, partly due to extra-nodular absorbed dose (AD) enhanced by residual TSH stimulation. <b>Objective</b>: We hypothesized that short-term LT3-induced TSH suppression at the time of RIT would promote long-term euthyroidism. <b>Patients and Methods</b>: A retrospective study was conducted on 95 UFA patients with SCH G1 (2001-2024). Patients underwent baseline and post-LT3 thyroid scintigraphy, and then received ¹³¹I-RIT with individualized dosimetry. Long-term bioclinical follow-up was achieved. <b>Results</b>: Short-term low-dose LT3 suppression caused no adverse events and significantly reduced TSH (0.45 to 0.047 mU/L). Whole-gland ¹²³I uptake decreased moderately (11.0 to 8.4%), while extra-nodular lobe uptake dropped markedly (1.77 to 0.73%) (all <i>p</i> < 0.0001). This focused activity on the UFA (2.5-fold increase), maintaining mean UFA AD (about 260 Gy) but reducing extra-nodular AD (61 to 37 Gy, <i>p</i> < 0.0001). Despite low ¹³¹I doses (mean 181 MBq), a dose-response relationship was observed: higher AD correlated with greater nodular lobe volume reduction (<i>p</i> < 0.033). At the 88-month follow-up, 93% of patients achieved normal thyroid function; one had persistent SCH G1, two were borderline hypothyroid, and two required LT4. <b>Conclusions</b>: ¹³¹I-RIT under brief LT3-induced TSH suppression induces sustained euthyroidism in SCH G1 with UFA. This simple, low-risk strategy reduces radioprotection concerns and is under evaluation to determine cardiovascular benefits.