PURPOSE: A seroma is a collection of exudates after surgical trauma in wound healing. Fibrin glue is used to prevent seroma by reducing the generation of exudate. However, the impact of fibrin glue on the prevention of seroma remains debatable. Therefore, we conducted a randomized controlled pilot trial to investigate the effect of the amount of fibrin glue used on the generation of exudate after thyroidectomy and the sample size of future definitive trials. METHODS: Between February and December 2020, 41 patients were enrolled; 21 patients in the low fibrin group and 20 in the high fibrin group. Stratified randomization was performed based on sex, body mass index, and thyroiditis. All patients underwent total thyroidectomy and bilateral central compartment dissection. In the low and high fibrin groups, 2 mL and 6 mL of fibrin glue were applied to patients, respectively. RESULTS: Both the total drain volume and flow rate during the first 12 hours were lower in the high fibrin group than in the low fibrin group (65.0 mL vs. 47.6 mL, P = 0.008 and 2.7 mL/hr vs. 1.8 mL/hr, P = 0.002, respectively). The calculated sample size for future randomized controlled trial was 32 patients (alpha = 0.05, power = 0.8), and the power of this trial was 0.91 with micro(1) = 2.7, micro(2) = 1.8, sigma = 0.9, and alpha = 0.05 (micro = mean, sigma = standard deviation). CONCLUSION: Six milliliters of fibrin glue could reduce total drain volume and flow rate of exudate after thyroidectomy. Therefore, applying an appropriate amount of fibrin glue after thyroidectomy may reduce postoperative seroma.