Graves' disease is the most common cause of hyperthyroidism. In this condition, the immune system sends signals to the thyroid, making it produce excessive amounts of thyroid hormone.
Other causes of hyperthyroidism are autonomous thyroid nodules (nodules that take over the function of the gland and produce excessive amounts of thyroid hormone) and multinodular toxic goiters (hyperactive nodules that produce too much thyroid hormone). Silent and subacute thyroiditis typically cause high thyroid hormone levels for a few weeks due to the release of thyroid hormone from the inflamed gland. Transient hyperthyroidism is typically followed by an underactive thyroid, which is then followed by the restoration of normal thyroid function. Many patients with subacute thyroiditis or silent thyroiditis will continue to have some thyroid hormone deficiency, often minimal, that can affect them.