Background Although the most common cause of thyrotoxicosis is Graves’ disease, the determination of the cause of thyrotoxicosis is important for establishing appropriate management. Diagnosis of surreptitious ingestion of thyroid hormones or factitious thyrotoxicosis often presents a difficult challenge especially in a patient with previously diagnosed Graves’ disease. The objective of this report was to demonstrate various approaches to support the diagnosis of factitious thyrotoxicosis. Case presentation We describe a patient with underlying Graves’ disease who underwent definitive therapy and needed long-term levothyroxine (LT4) replacement therapy. Later she developed thyrotoxicosis. Although factitious thyrotoxicosis was suspected because of very low thyroid uptake and low thyroglobulin (Tg) levels with the absence of thyroglobulin antibodies (TgAbs), she still refused any medication or substance use. After the administration of bile acid sequestrant, the thyroid hormone levels rapidly returned to normal within 1 month. Conclusions The diagnosis of factitious thyrotoxicosis is based upon the absence of goiter, suppressed serum Tg level, decreased radioactive iodine (RAI) uptake, and excellent response after cholestyramine treatment.