Graves' disease. Graves' disease is the most common cause of overactive thyroid. It can run in families and can occur at any age, although it is most common in women aged 20-40 years old.You are more likely to develop Graves' disease if you smoke. Graves' disease is an autoimmune condition.auses of overactive thyroid. Overactive thyroid (hyperthyroidism) occurs when your thyroid gland produces too much of the thyroid hormones thyroxine or triiodothyronine. Overproduction of thyroid hormones can be caused by a number of conditions, which are outlined below.