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

TSH with Reflex to Free T4

TSH first, with Free T4 run if needed, from one blood 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
No fasting. If you have had fluorescein angiography, wait 3 days.
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

This test starts with TSH, the pituitary’s signal to the thyroid. If TSH is above or below the lab’s reference range, the lab runs Free T4 on the same sample. TSH moves opposite to thyroid hormone, so the pair adds context. No fasting is needed; wait at least 3 days after fluorescein angiography.

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

Who should take this test?

This test is for adults starting a thyroid check, following up on a borderline TSH, or checking in on thyroid levels with a healthcare provider. 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

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 results show your TSH and, if it is out of range, your Free T4, which together can help a healthcare provider see whether your thyroid may be underactive or overactive.

Thyroid-stimulating hormone (TSH)
TSH is made by the pituitary gland at the base of your brain and tells your thyroid how much hormone to make. It usually moves in the opposite direction from thyroid hormone: TSH tends to rise when the thyroid is underactive and fall when it is overactive. It is the usual first thyroid check.
Why it’s measured: Shows how strongly your pituitary gland is signaling your thyroid to work.
Free thyroxine (free T4), if TSH is out of range
If your TSH result falls outside the lab’s reference range, the lab runs free T4 on the same sample. Free T4 is the unbound portion of the main thyroid hormone. Read together with TSH, it adds detail about how your thyroid is working.
Why it’s measured: Added by the lab only when TSH is outside the reference range.

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. If you have had fluorescein angiography, wait 3 days. 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

TSH is reported in mIU/L next to the lab’s reference range. If Free T4 is run, it appears in ng/dL with its own range.

  • Each marker 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 reported when TSH falls outside the lab’s range. A healthcare provider can explain your results and any next steps.

TSH, TSH with reflex, or a thyroid panel?

Three ways to start a thyroid check.

TSH with reflex

TSH first, with Free T4 run on the same sample when needed.

What the lab runs
TSH, then Free T4 if TSH is out of range
If TSH is out of range
The lab runs Free T4 on the same sample
Useful for
A first look that adds Free T4 when TSH calls for it

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 lab measures TSH first. If the result is above or below the lab’s reference range for your age and sex, the lab runs a Free T4 test on the same sample. If TSH is within range, Free T4 isn’t run.

Either way, the reflex step doesn’t need a second blood draw.

TSH is a sensitive signal, but it doesn’t show the thyroid hormone level itself. When TSH is high, Free T4 helps show whether an underactive thyroid is mild or more marked. When TSH is low, Free T4 helps a healthcare provider look at an overactive thyroid.

Other tests, such as Free T3 or antibodies, may be suggested after that.

It can. Fluorescein, a dye used in an eye exam called fluorescein angiography, can make results read falsely low. That is why the draw should wait at least 3 days after that exam, or 2 weeks for people on dialysis.

If you’ve had one recently, tell the healthcare provider who reads your results. Don’t stop or change any medicine or supplement without talking with your healthcare provider.

It looks at thyroid function, not its cause, and it is meant for times when a pituitary condition isn’t suspected. In some people, antibodies in the blood can interfere with the test. A healthcare provider reads your results with your symptoms and history and may suggest other tests.

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.