Normal thyroid levels range from 0.4 to 4 milli-international units per liter, and anything below this range is considered low, according to MedlinePlus. Low thyroid levels are often caused by an overactive thyroid or certain types of medication, including chemotherapy and opiate-based painkillers.
Thyroid tests measure the thyroid stimulating hormone in a person's blood, according to MedlinePlus. People who have an overactive or underactive thyroid disorder normally have their TSH levels kept between 0.5 and 4.0 milli-international units per liter. Some causes of low TSH are too much iodine caused by goiter, Grave's disease or imaging tests, such as a CT scan.