A 25 year old female presents with a 12 month history of palpitations, intermittent diarrhea, anxiety, and a 1-month history of "bulging of both eyes." What is the most likely cause of her symptoms?

Graves' disease is the most common cause of hypehyroidism in a young female and is the only one that causes exophthalmos ("bulging of both eyes"). Grave's disease is an autoimmune disorder in which a thyroid-stimulating IgG immunoglobulin (TSI) binds to the TSH receptors causing increased release of thyroid hormone. The exophthalmos is caused by lymphocytic infiltration of the extraocular muscles. Hashimoto's thyroiditis results in hypothyroidism and is associated with a diffusely enlarged thyroid gland and antimicrosomal antibodies against the thyroid parenchyma. Multinodular toxic goiter causes hypehyroidism, but does not result in exophthalmos. Papillary carcinoma will only very rarely present as a hypersecreting nodule. Most cases will be nonsecreting, cold nodules. There is no exophthalmos. This is the most common thyroid cancer and has the best prognosis of all thyroid cancers. Ref: Cooper D.S., Ladenson P.W. (2011). Chapter 7. The Thyroid Gland. In D.G. Gardner, D. Shoback (Eds), Greenspan's Basic & Clinical Endocrinology, 9e.