Thyroxine (T4) Free, Direct, S

A blood test that directly measures free T4 (thyroxine), the active form of thyroid hormone not bound to proteins. This is the "free" hormone that enters your body's tissues and regulates metabolism. Free T4 is the most accurate way to assess thyroid function because it is not affected by protein levels in your blood. Abnormal levels can indicate hypothyroidism (low T4) or hyperthyroidism (high T4). This test is often ordered alongside TSH to evaluate thyroid disorders and monitor treatment.