Graves' disease (German: Morbus Basedow), also known as toxic diffuse goiter, is an autoimmune disease that affects the thyroid. It frequently results in and is the most common cause of hyperthyroidism. It also often results in an enlarged thyroid. Signs and symptoms of hyperthyroidism may include irritability, muscle weakness, sleeping problems, a fast heartbeat, poor tolerance of heat, diarrhea and unintentional weight loss. Other symptoms may include thickening of the skin on the shins, known as pretibial myxedema, and eye bulging, a condition caused by Graves' ophthalmopathy. About 25 to 30% of people with the condition develop eye problems.

The exact cause of the disease is unclear; however, it is believed to involve a combination of genetic and environmental factors. A person is more likely to be affected if they have a family member with the disease. If one twin is affected, a 30% chance exists that the other twin will also have the disease. The onset of disease may be triggered by physical or emotional stress, infection, or giving birth. Those with other autoimmune diseases such as type 1 diabetes and rheumatoid arthritis are more likely to be affected. Smoking increases the risk of disease and may worsen eye problems. The disorder results from an antibody, called thyroid-stimulating immunoglobulin (TSI), that has a similar effect to thyroid stimulating hormone (TSH). These TSI antibodies cause the thyroid gland to produce excess thyroid hormones. The diagnosis may be suspected based on symptoms and confirmed with blood tests and radioiodine uptake. Typically, blood tests show a raised T₃ and T₄, low TSH, increased radioiodine uptake in all areas of the thyroid and TSI antibodies.

The three treatment options are radioiodine therapy, medications, and thyroid surgery. Radioiodine therapy involves taking iodine-131 by mouth, which is then concentrated in the thyroid and destroys it over weeks to months. The resulting hypothyroidism is treated with synthetic thyroid hormones. Medications such as beta blockers may control some of the symptoms, and antithyroid medications such as methimazole may temporarily help people while other treatments are having effect. Surgery to remove the thyroid is another option. Eye problems may require additional treatments.

Graves' disease will develop in about 0.5% of males and 3% of females. It occurs about 7.5 times more often in women than in men. Often, it starts between the ages of 40 and 60 but can begin at any age. It is the most common cause of hyperthyroidism in the United States (about 50 to 80% of cases). The condition is named after Irish surgeon Robert Graves, who described it in 1835. A number of prior descriptions also exist.