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Thyroid hormone: Free thyroxine (T4) concentration in serum/plasma

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

This test measures the amount of free thyroxine (T4), a hormone produced by your thyroid gland, in your blood. Free T4 is the active form of thyroid hormone that travels through your bloodstream to reach cells in your body.

Quick summary

["Your free T4 level helps doctors assess thyroid function and whether your thyroid is working properly."]

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

Key facts at a glance

LOINC
25791-5
Short code
T4 Free sp2 p chal SerPl-sCnc
Synonyms
T4 Free sp2 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 this test if you have symptoms of an overactive or underactive thyroid, such as weight changes, fatigue, or changes in your menstrual cycle. It may also be used to monitor thyroid hormone treatment or check for thyroid disorders like hypothyroidism or hyperthyroidism.

What Does a Free T4 Test Measure?

Free T4 measures the unbound (not protein-bound) thyroxine in your blood. Unlike total T4, which includes both free and protein-bound forms, free T4 reflects the hormone’s true biological activity. This is important because only free T4 can enter cells to regulate metabolism.

How Is the Test Performed?

A blood sample is taken from a vein in your arm. You may feel a slight pinch, but it usually doesn’t cause discomfort. The sample is sent to a lab for analysis using an immunoassay method to measure the free T4 concentration.

What Do Results Mean?

Results are not provided with specific reference ranges in this summary. Your doctor will interpret the results in relation to your symptoms, other tests, and medical history to determine if your thyroid function is normal or if treatment is needed.

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 causes changes in free T4 levels?
Changes in free T4 levels can be caused by thyroid disorders (such as hypothyroidism or hyperthyroidism), medications (like certain blood pressure or psychiatric medications), pregnancy, or conditions affecting thyroid hormone metabolism. Lifestyle factors, such as diet or stress, may also play a role.
How often should I get this test?
The frequency of testing depends on your medical condition and treatment plan. Your healthcare provider will determine how often you need this test based on your symptoms, lab results, and overall health. Regular monitoring may be necessary if you have a thyroid disorder or take thyroid hormone medication.
Can lifestyle changes affect my free T4 test results?
Yes, certain lifestyle factors—such as diet (e.g., iodine intake), stress, sleep, and exercise—can influence thyroid function. However, free T4 levels are primarily interpreted in the context of your overall health and medical history, not just lifestyle alone.
  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