Free Triiodothyronine (FT3) Blood Test

A free or total triiodothyronine (free T3 or total T3) test is used to assess thyroid function. This test measures the level of a hormone called triiodothyronine in your blood. T3 is produced by your thyroid in response to thyroid stimulating hormone (TSH), which is created by the pituitary gland. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor treatment of a person with a known thyroid disorder.

Normal range: 130 to 450 pg/dL, or 2.0 to 7.0 pmol/L. T3 test results are often compared with T4 and TSH test results to help diagnose thyroid disease.

Low T3 levels. Low T3 levels may mean you have hypothyroidism, a condition in which your body doesn't make enough thyroid hormone. The most common cause of hypothyroidism is an autoimmune disorder known as Hashimoto's thyroiditis. Autoimmune disorders occur when your immune system produces antibodies that attack your own tissues. Sometimes this process involves your thyroid gland. Besides, if you have a long-term illness it may low your T3 levels since T3 levels decrease when you're sick. If you're sick enough to be hospitalized, your T3 levels are likely to be low.

High FT3 levels. If your results show high total T3 levels or high free T3 levels, it may mean you have hyperthyroidism. Hyperthyroidism can be caused by a number of conditions, including Graves' disease, Plummer's disease and thyroiditis.