Lugol's solution is an iodinated agent used for treating thyroid crisis. It is primarily used in diagnostic tests for esophageal diseases. However, Lugol's solution can cause local mucosal injury and hemorrhage. We report, for the first time, a case of 34-year-old man who exhibited severe duodenal hemorrhage induced by Lugol's solution that was used to treat thyroid crisis. The quantity of Lugol's solution used for treating thyroid crisis is much higher than that used for mucosal disease investigation. Clinical practitioners should be aware of gastrointestinal hemorrhage when using Lugol's solution for the treatment of thyroid crisis.