About Thyroxine (T4) Free
Free Thyroxine (T4) is an unbound form of the thyroxine hormone, produced by the thyroid gland. It plays a critical role in metabolism, growth, and development, affecting nearly every tissue in the body. Free T4 is a valuable test as it provides a more accurate reflection of thyroid function compared to total T4, which includes both bound and unbound thyroxine.
Reference Values
Optimal Range
0.8 - 1.8 ng/dL
Women's Optimal Range
0.9 - 1.7 ng/dL
Elite Value
Not specified
Health Implications
High
Elevated levels may indicate hyperthyroidism, which can lead to symptoms such as increased heart rate, anxiety, and weight loss.
Low
Low levels may be associated with hypothyroidism, resulting in fatigue, weight gain, and depression.
Symptoms
High
May include anxiety, unexplained weight loss, increased heart rate, and excessive sweating. Symptoms might also include tremors, difficulty sleeping, and irritability.
Low
Can include fatigue, depression, weight gain, cold intolerance, dry skin, and constipation. Other potential symptoms are memory issues and slowed heart rate.
Lifestyle Factors
Specimen Type
Testing Methods
Also Known As
Scientific Sources
- 1. Ross, D. S., Burch, H. B., Cooper, D. S., Greenlee, M. C., Laurberg, P., Maia, A. L., ... & Walter, M. A. (2016). 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. *Thyroid*, 26(10), 1343-1421. 2. Garber, J. R., Cobin, R. H., Gharib, H., Hennessey, J. V., Klein, I., Mechanick, J. I., ... & Woeber, K. A. (2012). Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. *Thyroid*, 22(12), 1200-1235. 3. Biondi, B., & Wartofsky, L. (2014). Treatment with thyroid hormone. *Endocrine Reviews*, 35(3), 433-512. 4. Gaitonde, D. Y., Rowley, K. D., & Sweeney, L. B. (2012). Hypothyroidism: an update. *American Family Physician*, 86(3), 244-251. 5. American Thyroid Association. (2014). Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. *Thyroid*, 24(12), 1670-1751.