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Free thyroxine (T4) concentration in serum/plasma

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

This test measures the amount of active thyroid hormone (T4) that is not bound to proteins in your blood. Free T4 helps your body use energy and regulate metabolism.

Quick summary

["Free T4 reflects your active thyroid hormone levels independently of protein binding."]

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

Key facts at a glance

LOINC
25795-6
Short code
T4 Free sp6 p chal SerPl-sCnc
Synonyms
T4 Free sp6 p chal SerPl-sCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
4.5
Ref high
12
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84436
SNOMED
302345006
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 to check for thyroid function, especially if you have symptoms like fatigue, weight changes, or unexplained hair loss. It’s often used alongside thyroid-stimulating hormone (TSH) tests to diagnose or monitor thyroid disorders, such as hypothyroidism or hyperthyroidism.

How is the sample collected?

A blood sample is taken from a vein in your arm. You may feel a small pinch when the needle is inserted, but the procedure is quick and usually doesn’t cause discomfort afterward.

What should I expect before or after the test?

No special preparation is needed for this test. You can eat and drink normally. If you’re on thyroid medication, let your provider know before your appointment. After the blood draw, apply gentle pressure to the site to stop bleeding and avoid lifting your arm above your head for a short time.

What does the test result mean?

Results help determine whether your thyroid is functioning normally or if there’s an imbalance. Your provider will interpret the results in relation to your symptoms and other tests.

Reference range

Adult: 4.5-12.0 μg/dLN/A

Specimen: serum

Take your thyroid medication after the blood draw if applicable. No fasting required.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Free T4

Frequently asked questions

What does a low free T4 level mean?
A low free T4 level may indicate hypothyroidism, where your thyroid isn’t producing enough active hormone. This can lead to symptoms like fatigue, weight gain, or cold sensitivity. Your provider may recommend treatment or further testing.
How does free T4 differ from total T4?
Free T4 is the portion of thyroid hormone that’s not bound to proteins in your blood, making it the active form that your body uses. Total T4 includes both free and bound hormone, so free T4 gives a clearer picture of your thyroid’s true activity.
Can diet affect free T4 levels?
Dietary factors like iodine intake or certain medications (e.g., soy products, thyroid medications) can influence thyroid function. Always discuss your diet and medications with your provider before interpreting test results.
  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