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.
A bacterial or viral infection can make your thyroid gland swell. This is called thyroiditis. The damaged thyroid leaks its hormone into your blood, which causes your thyroid hormone level to rise for a short time. Once that hormone is used up, your thyroid hormone levels will drop. Low levels from thyroiditis often don't last long, because the gland hasn't been permanently damaged.