Skip to content

Free thyroxine (T4) in blood mass concentration

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

This test measures the amount of free thyroxine (T4), a hormone produced by your thyroid gland that plays a key role in regulating metabolism.

Quick summary

["Free T4 helps assess thyroid function and identify potential hormone imbalances."]

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

Key facts at a glance

LOINC
73557-1
Short code
T4 Free BldCo-mCnc
Synonyms
T4 Free BldCo-mCnc
Category
chemistry
Sample
whole-blood
Container
Lavender-top (EDTA)
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
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

Why Is This Test Done?

Your healthcare provider may order a free T4 test to check for thyroid disorders, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It’s often used alongside thyroid-stimulating hormone (TSH) tests to get a clearer picture of your thyroid health.

What Does a Free T4 Test Tell You?

Free T4 levels help determine whether your thyroid is producing enough or too much hormone. Unlike total T4, which includes bound hormone, free T4 reflects the active, biologically available portion of the hormone in your blood.

How Is the Test Performed?

A blood sample is drawn from a vein in your arm. The sample is processed to measure the free T4 concentration using a specialized laboratory technique, such as immunoassay or chemiluminescent immunoassay.

What Are the Results?

Results are typically reported as a mass concentration value. Your healthcare provider will compare these results to standard reference ranges to interpret whether your levels are within normal limits or if further evaluation is needed.

Reference range

Adult: 4.5-12.0 μg/dLN/A

Specimen: whole-blood

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 free T4 test measure?
This test measures the amount of free (unbound) thyroxine hormone circulating in your blood, which is the active form that affects your body’s metabolism and other functions.
Why might my healthcare provider order a free T4 test?
Your provider may order it to evaluate thyroid function, especially if you have symptoms like unexplained weight changes, fatigue, or changes in heart rate, or if they suspect thyroid disease.
Can free T4 levels change with diet or medications?
Yes, certain medications (like thyroid hormone replacements or some antidepressants) and dietary factors (such as iodine intake) can influence thyroid hormone levels, including free T4. Always discuss any changes in your medications or diet with your healthcare provider.
  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