Free T4
Also called Free thyroxine, FT4
Free T4 (free thyroxine) is the portion of thyroxine circulating in the blood that is not attached to a carrier protein and can enter the body's tissues. Thyroxine, or T4, is the main hormone the thyroid gland makes; most of it travels bound to proteins as a reserve, and only the free fraction is available to act.
A free T4 test is generally preferred over a total T4 test, which counts bound and free hormone together, because only the free portion reaches the tissues and medical experts consider it the more accurate measure. On its own a T4 result cannot diagnose a thyroid problem, so it is read with TSH: a high TSH with a low free T4 points one way, a low TSH with a high free T4 the other. How the three thyroid markers are read together is explained in TSH, Free T4 and Free T3: What Each One Measures.
Why it matters after 45
Hypothyroidism becomes more common with age, particularly in women, and its slow onset means symptoms such as tiredness, weight gain and low mood can build for months before anyone tests. Free T4 also moves for reasons that are not thyroid disease: estrogen-containing medicines such as oral contraceptives raise thyroid hormone levels, while serious illness and corticosteroid medicines can lower T4. Both are reasons a midlife result is interpreted against current illness and the medicines being taken.
Where it comes up
- Is it midlife, or is it your thyroid?thyroid symptoms that overlap with the menopause transition
- Normal TSH but still exhaustedwhen the first thyroid result does not explain the symptoms
- Oral GLP-1s and thyroid medicationwhy levothyroxine absorption is monitored
- Comprehensive Lab Panelthe baseline panel that includes thyroid markers
References
- MedlinePlus. Thyroxine (T4) Test. U.S. National Library of Medicine. https://medlineplus.gov/lab-tests/thyroxine-t4-test/
- National Institute of Diabetes and Digestive and Kidney Diseases. Thyroid Tests. https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid
- National Institute of Diabetes and Digestive and Kidney Diseases. Hypothyroidism (Underactive Thyroid). https://www.niddk.nih.gov/health-information/endocrine-diseases/hypothyroidism
Definitions here are educational and are not medical advice. They explain what a term means; the pages linked above cover what it means for you, and a clinician reviews your own results. Editorial & medical review policy · All glossary terms