BACKGROUND: Apium graveolens is one of the well-known herbs used for the treatment of different; however, allergic reactions have been reported after its use. This report aimed to demonstrate the A. graveolens induced hyperthyroidism after its oral consumption for weight loss. CASE REPORT: Mr. A, 48-year-old, with no history of any thyroid diseases, was diagnosed with hyperthyroidism due to daily consumption of 4 g of dried celery leaves for 45 days. After cessation of consumption and treatment with methimazole, the symptoms remitted. Then, the medication was discontinued when the lab tests and ultrasound were normal and indicated the patient’s definite recovery. In 2 months follow up of, he was normal and thyroid-stimulating hormone (TSH), T4, T3, anti-TSH receptor, anti thyroperoxidase and antithyroglobulin were in normal ranges. CONCLUSION: Hyperthyroidism may be induced by consumption celery. Although many studies have reported side effects such as allergic reactions for this herb, this is the first report of hyperthyroidism induced by celery in which the patient recovered after discontinuing the medication. Therefore, it can be assumed that celery induces hyperthyroidism as a side effect of this herb if it is used for a long term.