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

Free T3

The unbound T3 thyroid hormone in your blood, 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

T3 is a thyroid hormone, and much of it is made from T4 in the body’s tissues. Free T3 measures the portion that is not bound to carrier proteins. It is mainly read with TSH and Free T4 for overactive thyroid questions, and it is less useful for an underactive thyroid. No fasting needed.

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

Who should take this test?

This test is mostly used after a low TSH or with signs of an overactive thyroid, and it is read with TSH and Free T4 by a healthcare provider. Commonly related symptoms include:

Heart

  • Racing heartbeat

Weight

  • Unexplained weight loss

Body

  • Feeling hot or sweaty
  • Shaky hands

Mood

  • Feeling anxious or on edge

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 T3 is in your blood, which, read with TSH and Free T4, can help a healthcare provider understand whether your thyroid may be overactive.

Free triiodothyronine (free T3)
T3 is a thyroid hormone made by the thyroid and from T4 in other tissues. Free T3 is the small portion that is not bound to proteins. It is mainly used, together with TSH and free T4, when an overactive thyroid is a question, and it can shift during illness.
Why it’s measured: Adds detail mainly when an overactive thyroid is a question.

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 T3 is reported in picograms per milliliter (pg/mL) 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

A Free T3 outside the range is read with TSH and Free T4. A healthcare provider can explain your result and any next steps.

Free T3 or total T3?

Two ways to measure the same hormone.

Free T3

The T3 that is not bound to carrier proteins.

Measures
The unbound portion of T3
Carrier proteins
Less affected by changes in them
Mainly used for
Overactive thyroid questions, read with TSH and Free T4

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.

The thyroid releases mostly T4, and the body converts some of it into T3 in its tissues. T3 is the form that acts on cells. Free T3 and Free T4 each measure the unbound portion of their hormone.

Free T4 is the usual follow-⁠up to an abnormal TSH. Free T3 is mostly added when an overactive thyroid is the question.

In an underactive thyroid, Free T3 may not drop until well after TSH and Free T4 have changed. That makes it less helpful for that question, so TSH and Free T4 are usually checked first. A healthcare provider reads Free T3 together with those results.

A healthcare provider reads it with TSH and Free T4, your symptoms, and any thyroid medicine you take. They may suggest repeating the tests or adding others before reaching a conclusion. Don’t change any thyroid medicine based on this result alone; talk with your prescriber.

No. This test is not for emergencies. If you have chest pain, a racing or irregular heartbeat that does not settle, fainting, trouble breathing, or sudden confusion, get emergency care right away rather than waiting for a lab visit or a result.

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.