Hyperthyroidism, or an overactive thyroid, is caused by excessive secretion of thyroid hormones from the thyroid gland. Common causes include Graves' disease, toxic nodular goiter, and thyroiditis. Symptoms include rapid heart rate, sweating, weight loss, and anxiety. Diagnosis involves blood tests to measure thyroid hormone levels. Treatment options are anti-thyroid medications, radioactive iodine therapy, or surgery to remove part of the thyroid gland.