Abstract： Objective:To screen the factors that were conducive to prolonging the effective half-life of 131I, and to provide a basis for selecting more suitable patients for the treatment of hyperthyroidism with lithium carbonate assisted radioactive 131I. Methods:Between July 2017 and March 2019, a total of 61 patients (23 males, 38 females, age: 13-73 (37.7±1.9) years) who received lithium carbonate adjuvant therapy before 131I treatment in the First Affiliated Hospital of Zhengzhou University were collected into this prospective study. Clinical parameters (serum free triiodothyronine (FT 3), serum free thyroxine (FT 4), iodine uptake rate (2 h, 12 h, 24 h) and ratio of iodine uptake rate (2 h/24 h, 4 h/24 h) before and after taking lithium carbonate were compared by using paired t test. The factors which were conducive to prolonging the effective half-life of 131I were analyzed by linear and multiple regression analyses. Results:Compared with the pretreatment results, lithium carbonate treatment significantly improved the 24 h iodine uptake rate ((74.82±2.69)% vs (82.38±2.33)%; t=2.674, P=0.010), decreased the ratio of 4 h/24 h iodine uptake rate ((88.96±2.85)% vs (82.12±2.27)%; t=2.644, P=0.010), prolonged the effective half-life of 131I ((123.03±3.09) vs (130.38±2.49) h; t=2.656, P=0.010), and decreased serum FT 3 ((26.46±1.65) vs (21.31±1.42) pmol/L; t=3.421, P=0.001) and serum FT 4 ((56.59±2.12) vs (49.24±2.19) pmol/L; t=3.289, P=0.002). Linear regression analysis showed that 2 h and 4 h iodine uptake rate, the ratios of 2 h/24 h and 4 h/24 h iodine uptake rate, and effective half-life of 131I before lithium carbonate treatment were related to the effective half-life of 131I after lithium carbonate treatment ( b values: from -0.576 to 0.642, all P<0.05). Multiple linear regression analysis showed that the ratio of 4 h/24 h iodine uptake rate was the main factor affecting the effective half-life of 131I after taking lithium carbonate ( b=0.642, 95% CI: 0.453-0.832, P<0.001). Multiple linear regression equation was y=-49.785+ 0.642 x. Lithium carbonate treatment could prolong the effective half-life of 131I if the ratio of 4 h/24 h iodine uptake exceeded 77.55%. Conclusion:The ratio of 4 h/24 h iodine uptake rate is the main factor affecting the change of the effective half-life of 131I after lithium carbonate treatment, and patients with hyperthyroidism whose ratio of 4 h/24 h iodine uptake rate exceeds 77.55% can effectively prolong the effective half-life of 131I after taking lithium carbonate.