Too much thyroid hormone can cause problems. This is known as Hyperthyroidism. So what causes Hyperthyroidism? Well, according to the AAFP (American Academy of Family Physicians), the most common cause of Hyperthyroidism is a disease called Grave’s disease. In fact, it is responsible for about 50-80% of all cases usually seen in women between the ages of 30 to 50. This disease is seen 4-10 times more in women than in men.