What are the blood tests used to detect low thyroid hormone?
The “T4” and “TSH” blood tests are the two tests typically used to diagnose low thyroid hormone levels. T4 is a type of thyroid hormone; a test called the “free T4” is most commonly used to measure T4. Total T4 is another test that is sometimes used. A total T4 less than 4.6 mg/dL and a free T4 less than 0.5 ng/dL are considered low and indicate an underactive thyroid gland. T3 is another type of thyroid hormone that may also be measured under certain circumstances. However, when the thyroid slows down, the T4 level usually falls before T3 levels. Therefore, T3 levels rarely need to be measured. A TSH (Thyroid Stimulating Hormone) blood test is also used to detect an underactive thyroid condition. TSH is a substance that is produced by the brain. It tells the thyroid gland how much thyroid hormone to produce. When thyroid hormone is low, the brain releases more TSH into the blood stream to stimulate the thyroid gland. In other words, TSH levels rise when thyroid hormone is low. A high
