We describe a case of thyroid storm due to thyrotoxicosis factitia, which was caused by the ingestion of excessive quantities of exogenous thyroid hormone for the purpose of reducing weight. An 18-year-old female was admitted to the hospital 24 hours after taking up to 50 tablets of synthyroid (1 tablet of synthyroid : levothyroxine 100 µg). Because of her stuporous mental state and acute respiratory failure, she was intubated and treated in the intensive care unit. After reviewing her history carefully and examining plasma thyroid hormone levels, we diagnosed this case as a thyroid storm due to thyrotoxicosis factitia. Her thyroid function test revealed that T3 was 305 ng/dL, T4 was 24.9 µg/dl, FT4 was 7.7 ng/dL, TSH was 0.05 µIU/mL and TBG was 12.84 µg/mL (normal range: 11.3 - 28.9). TSH receptor antibody, antimicrosomal antibody, and antithyroglobulin antibody were negative. She was recovered by treatment, namely, steroid and propranolol, and was discharged 8 days after admission. Thyroid storm due to thyrotoxicosis factitia caused by the ingestion of excessive thyroid hormone is rarely reported worldwide. Therefore, we now report a case of thyroid storm that resulted from thyrotoxicosis factitia caused by the ingestion of a massive amount of thyroid hormone over a period of 6 months.