What it is: A T4 (thyroxine) test measures the level of thyroxine, the main hormone the thyroid gland releases, in the blood — as either total T4 or free T4.
Why it's ordered: Ordered to follow up an abnormal TSH result, diagnose hyperthyroidism or hypothyroidism, or evaluate autoimmune thyroid conditions like Graves' or Hashimoto's disease.
What it detects: T4 (with T3) regulates metabolic rate, heart and digestive function, and other body processes. Measuring T4 alongside TSH gives a fuller picture of thyroid function than TSH alone.
Collection & prep: A standard blood draw from a vein in the arm, usually alongside a TSH test. No special preparation is typically required.
Results:
High – suggests hyperthyroidism (overactive thyroid)
Normal – interpreted alongside TSH to confirm overall thyroid status
Possible complications of thyroid dysfunction: Providers often prescribe synthetic levothyroxine to treat low T4 from hypothyroidism; abnormal T4 combined with abnormal TSH usually leads to further testing, such as thyroid antibodies, to identify the underlying cause.
Statements made in the Clinical Manual have not been evaluated by the FDA. The products mentioned are not intended to diagnose, treat, cure or prevent any disease.
