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.
Full Removal. If your doctor removes your entire thyroid, your body will begin to exhibit symptoms similar to hypothyroidism, due to the lack of thyroid hormones in your body. These symptoms may include fatigue, elevated cholesterol levels, uncontrollable weight gain, muscle weakness and depression.
The second major cause is the broad category of medical treatments. The treatment of many thyroid conditions warrants surgical removal of a portion or all of the thyroid gland. If the total mass of thyroid producing cells left within the body are not enough to meet the needs of the body, the patient will develop hypothyroidism.