Graves’ disease is an autoimmune disease that leads to a generalized overactivity of the entire thyroid gland (hyperthyroidism). It is the most common cause of hyperthyroidism in the United States. It is named after Robert Graves, an Irish physician, who described this form of hyperthyroidism about 150 years ago. It is 7-8 times more common in women than men.
Source: American Thyroid Association
In the US, it affects around 3% of adults, and produces a variety of symptoms, affecting : growth and development, body temperature, heart rate, weight, fertility, sleep patterns etc. It often appears suddenly with no apparent cause.
It's known that the overactive thyroid is triggered by antibodies called thyroid-stimulating immunoglobulins (TSIs). The TSIs bind to thyroid cell receptors, causing the thyroid to grow and release excess thyroid hormone.
But the factors which trigger the 'faulty' immune response are unknown, it's believed to involve a combination of genetic and environmental factors - including infections and stress. (It's known to 'run in families' but to date, no clear genetic defect has been found to point to a single-gene cause.)