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Serum or plasma concentration of free thyroxine (T4) by immunoassay

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of free thyroxine (T4), the active form of thyroid hormone circulating in your blood. It helps assess your thyroid function and whether your thyroid is producing enough hormone.

Quick summary

["Free T4 levels help diagnose and monitor thyroid disorders like hypothyroidism or hyperthyroidism."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
77479-4
Short code
Deprecated Hind+Mid ft-L XR
Synonyms
Deprecated Hind+Mid ft-L XR
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
xr
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why Is This Test Done?

Your healthcare provider may order a free T4 test if you have symptoms of thyroid problems, such as fatigue, weight changes, or unexplained mood swings. It’s also used to check thyroid function in people with known thyroid conditions or during pregnancy.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 12 hours before the blood draw if your test is part of a thyroid panel. Avoid smoking or caffeine for 30 minutes before the test.

What Happens During the Test?

A healthcare professional will draw blood from a vein in your arm. The blood sample is sent to a lab for analysis using an immunoassay method to measure free T4 levels.

What Are the Results?

Results are typically reported as a concentration value (in ng/dL or similar units). Your results will be compared to reference ranges provided by your healthcare provider to determine if your thyroid function is within normal limits.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Free T4

Frequently asked questions

What causes abnormal free T4 levels?
Abnormal free T4 levels can result from thyroid diseases (e.g., Hashimoto’s thyroiditis, Graves’ disease), thyroid nodules, pituitary disorders, or medications that affect thyroid hormone levels. Pregnancy, stress, or certain medical conditions may also influence results.
How often should I get this test?
Your provider will determine how often you need retesting based on your symptoms, medical history, and treatment plan. Regular monitoring may be needed if you have a thyroid condition or take thyroid-replacement medication.
Can lifestyle changes affect free T4 test results?
Yes, certain lifestyle factors like diet (e.g., iodine intake), smoking, caffeine, or stress can temporarily affect thyroid hormone levels. Your provider may advise avoiding these influences before testing if needed.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy