A free triiodothyronine (free T3) test is used to assess thyroid function in a patient. It is ordered to help diagnose hyperthyroidism and may be ordered to help monitor treatment of a person with a known thyroid disorder. T3 (triiodothyronine) is one of two major hormones produced by the thyroid, a small, butterfly-shaped gland located at the base of the throat. T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.