A patient seated in the chair before a blood draw
In person

Free T4

The unbound portion of the thyroid hormone T4, from one draw.

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  • 2,000+ lab locations. Find one near you
  • Nothing due at the lab. FSA / HSA accepted
  • Ordered by a licensed clinician

At a glance

Collection
Blood draw
A standard blood draw at the lab.
Preparation
None
Nothing to do ahead of time. Water is fine.
Results
1 to 3 days
Typical turnaround after your visit.
Where
2,000+ lab locations
Nationwide, with nothing due at the lab.

About this test

T4 (thyroxine) is the main hormone the thyroid releases. Most T4 in the blood is attached to carrier proteins. Free T4 measures the portion that is not bound, which is available to your cells. It is usually read with TSH to check thyroid function, and no fasting is needed.

For adults 18 and older. Test availability varies by lab location.

Who should take this test?

This test is often used to follow up on a TSH result or to check thyroid levels with a healthcare provider, including while taking thyroid medicine. Commonly related symptoms include:

Body

  • Tiredness
  • Feeling cold or too warm

Weight

  • Unexplained weight change

Hair

  • Hair thinning

Skin

  • Dry skin

Neck / thyroid

  • Swelling in the neck

Family history

  • Thyroid conditions in family

Ask about this test

Not sure this is the right test?

AI-generated. For information only, not medical advice. Call 911 for an emergency.

What does this test measure?

Your result shows how much unbound T4 is in your blood, which, read with TSH, can help a healthcare provider understand whether your thyroid may be underactive or overactive.

Free thyroxine (free T4)
Thyroxine (T4) is the main hormone your thyroid makes. Most of it travels bound to proteins in the blood; free T4 is the small unbound portion available to your tissues. It is read with TSH to help show whether the thyroid is making too little or too much hormone.
Why it’s measured: The unbound portion of the main thyroid hormone, read with TSH.

How it works

  1. Step 1

    Order online

    Check out here. A licensed clinician in your state reviews your order and sends it to the lab.

  2. Step 2

    Visit a lab

    Bring a photo ID to any of 2,000+ lab locations. No fasting needed, and nothing is due at the lab.

  3. Step 3

    Get your results

    Results post to your account, usually in 1 to 3 days, each number next to the lab’s reference range.

Find a lab near you

Choose from 2,000+ lab locations nationwide. Your order works at any of them.

What your results tell you

Free T4 is reported in nanograms per deciliter (ng/dL) and shown next to the lab’s reference range for comparison.

  • Your number next to the lab’s reference range
  • Explained in plain language
  • A licensed clinician reviews your order and, if appropriate, orders the test.
  • A lab report you can download and share

Free T4 is read with TSH and your symptoms. A healthcare provider can explain your result; don’t change thyroid medicine based on it alone.

Free T4 or total T4?

Two ways to measure the same hormone.

Free T4

The T4 that is not bound to carrier proteins.

Measures
The unbound portion of T4
Carrier proteins
Less affected by changes in them
Mainly used for
Following up on TSH and checking thyroid function

Clinical-grade testing, from the comfort of home.

From the lab to your report, every step of our process meets the gold standard in medical testing.

  • CLIA-certified labs

    Every lab we partner with is certified under the Clinical Laboratory Improvement Amendments, meeting both state and federal standards with regular independent inspections.

  • We protect your health data

    We protect your health data with encryption and do not sell it. Some results must be reported to public health authorities by law. See our Privacy Notice.

  • Rigorous quality assurance

    All partner labs run regular internal quality checks, participate in proficiency testing, and comply with federal and state lab regulations.

  • Clinician-reviewed reports

    Results are reviewed by an independent, licensed clinician.

Questions

Common questions.

Anything else, our care team answers by phone, text, or email.

Most T4 travels in the blood attached to proteins. Changes in those proteins can move total T4 up or down even when thyroid function hasn’t changed. Free T4 is less affected by this, which is why it has largely replaced total T4 for many thyroid checks.

TSH is the pituitary’s signal to the thyroid. When thyroid hormone is low, TSH tends to rise, and when it is high, TSH tends to fall.

Reading TSH with Free T4 helps a healthcare provider see whether the thyroid may be underactive or overactive, and whether the signal and the hormone level line up.

Free T4 shows the level of one thyroid hormone at the time of the draw. It does not show the cause of a high or low result, and it is not an antibody test. Very unusual carrier proteins can also affect this method, so a healthcare provider may use a different test in some cases.

Lab tests are informational and don’t diagnose conditions on their own. Available to adults 18 and older; test availability varies by lab location. A licensed clinician reviews your order and, if appropriate, orders the test. All testing is performed at CLIA-certified laboratories. Not for emergencies: if you think you may be having a medical emergency, call 911.