The main thyroid hormone produced by your thyroid — the precursor that converts to active T3.
What is it
Free T4 (thyroxine) is the primary hormone produced by the thyroid gland and unbound to carrier proteins. It is converted to the active T3 in peripheral tissues.
Why it's measured
Free T4 is a foundational marker in thyroid evaluation, used alongside TSH to characterize thyroid status in research and clinical practice.
Reference range
Standard laboratory reference
0.8 – 1.8 ng/dL
Reference ranges vary by laboratory. Your individual reference range will appear on your test report and should be interpreted by your physician in the context of your overall health profile.
Where does your number land?
—
Drag to your own Free T4 result.
Educational only, not a diagnosis. Ranges vary by laboratory. See which panel includes Free T4.
What results may indicate
Elevated
Has been associated with hyperthyroidism in published research.
Lower
Has been associated with hypothyroidism in published research.
These associations are general. They are not a personal diagnosis or prediction. Discuss your individual results with your physician in the context of your full health profile.
When it's measured
Routinely measured alongside TSH in comprehensive thyroid evaluation.
Related markers
Common questions
Free T4 (thyroxine) is the primary hormone produced by the thyroid gland and unbound to carrier proteins. It is converted to the active T3 in peripheral tissues.
It's one of the markers used to build a fuller picture of your health. On its own it tells you very little, so it's most informative when reviewed alongside related markers.
It's measured from a standard blood draw at Quest or LabCorp. Free T4 is one of the markers included in the OPTML Apex Panel, where your results are read across systems and reviewed by a U.S.-licensed physician.
Reference ranges vary by laboratory, and a single value rarely tells the whole story. Review your result with a physician in the context of your other markers, symptoms, and history.
Free T4 is one signal among many. Answer a few quick questions and we’ll point you to the panel that fits.
Measured in
A complete look at your heart, hormones, metabolism, inflammation, and longevity systems — drawn at Quest or LabCorp, reviewed by a U.S.-licensed physician.
Find the right panel →This information is for general educational purposes only and is not medical advice. Lab results alone are not intended to diagnose, treat, or cure any condition and do not replace the advice of a healthcare provider. OPTML does not offer medical advice, a diagnosis, medical treatment, or any form of medical opinion.
OPTML offers compounded semaglutide and tirzepatide, prescribed and overseen by U.S.-licensed physicians. Compounded drug products are not approved or evaluated for safety, effectiveness, or quality by the FDA. OPTML is not associated with, endorsed by, or affiliated with the manufacturers of FDA-approved branded weight-loss medications, and does not sell or supply those branded products. Anyone interested in an FDA-approved branded medication should consult a licensed healthcare provider or pharmacist. Individual results vary; no specific result is promised, and weight management requires ongoing medical guidance.
The whole picture in one sitting: who it suits, what the labs decide, what the first year actually looks like. Every clinical claim is cited.
How to protect lean mass while losing weight on a GLP-1.
Double-check your email. A typo means we cannot reach you or resend your guide.
We will send occasional emails you can unsubscribe from at any time. See our Privacy Policy.