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Free T4 test

Also called: Free thyroxine, FT4

A free T4 test measures the active thyroid hormone in your blood. It's used with TSH to diagnose and treat thyroid problems.

SampleA small tube of blood from a vein in your arm
Fasting?Usually not needed
ResultsUsually within 1 to 2 days
Good to knowOne result rarely tells the whole story. Your provider looks at your results together with your symptoms, exam, and other tests.

What the test is

Biotin supplements can interfere with results.

Why it's done

  • Abnormal TSH
  • Monitoring thyroid treatment
  • Pituitary problems

Getting ready and what happens

  • Ask about stopping biotin 2 to 3 days before.

How the test is done

  • A health worker cleans the skin, puts a band around your arm, and draws blood from a vein with a small needle. It takes a few minutes.

What the results mean

ResultTypical range for adults
Free T4About 0.8 to 1.8 ng/dL (labs vary)

Normal ranges vary from lab to lab and can differ by age, sex, and pregnancy. Your report lists your lab's own range next to each result. A result a little outside the range is often not a problem.

Low free T4 with high TSH

Points to an underactive thyroid.

High free T4 with low TSH

Points to an overactive thyroid or too much thyroid medicine.

Questions to ask your provider

  • What do my results mean for me?
  • Do I need to repeat this test or have other tests?
  • What do my TSH and free T4 mean together?

Risks

A blood draw is very safe. You may have slight pain or a small bruise where the needle went in.

Related conditions

Related medicines

Related tests

Sources

Last updated 2026-11-13.