A TSH test measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland. It prompts the thyroid gland to make and release thyroid hormones into the blood. A higher-than-normal TSH level is most often due to an underactive thyroid gland (hypothyroidism). There are many causes of this problem. A lower-than-normal level may be due to an overactive thyroid gland, which can be caused by: Graves disease Toxic nodular goiter or multinodular goiter Too much iodine in the body (due to receiving iodine contrast used during imaging tests, such as CT scan) Taking too much thyroid hormone medicine or prescribed natural or over-the-counter supplements that contain thyroid hormone Use of certain medicines may also cause a lower-than-normal TSH level. These include glucocorticoids/steroids, dopamine, certain chemotherapy drugs, and opioid painkillers such as morphine.