Skip to content

Free thyroxine (T4) concentration in blood

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, which can help assess your thyroid function.

Quick summary

["Free T4 helps determine if your thyroid is producing enough active hormone for your body’s needs."]

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

Key facts at a glance

LOINC
98182-9
Short code
T4 Free BldCo-sCnc
Synonyms
T4 Free BldCo-sCnc
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 This Test Might Be Ordered

Your healthcare provider may order a free T4 test if you have symptoms like fatigue, weight changes, or unexplained hair loss, or if you have conditions such as hypothyroidism or hyperthyroidism. It’s also used to monitor thyroid hormone levels during treatment for thyroid disorders.

What to Expect Before the Test

No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the draw if your blood is collected in the morning. Wear a comfortable shirt for easy access to your arm.

How the Test Works

A small amount of blood is drawn from a vein in your arm. The sample is then analyzed in a lab to measure the free T4 concentration, which is separated from protein-bound hormone.

What the Results Mean

Results are compared to a general reference range, but individual ranges can vary by lab. Low free T4 levels may suggest hypothyroidism, while high levels could indicate hyperthyroidism or other conditions affecting thyroid function.

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

How often should I get this test?
The frequency depends on your health status. If you have thyroid disease, your provider may recommend testing every few months to monitor your treatment. Otherwise, it’s usually ordered as needed for symptoms or diagnostic purposes.
What does a low free T4 mean?
A low free T4 level often indicates hypothyroidism, where your thyroid isn’t producing enough hormone. This can cause symptoms like fatigue, cold intolerance, or weight gain. Your provider may adjust your medication or further investigate underlying causes.
Can diet affect free T4 test results?
Dietary supplements like soy, selenium, or certain medications (e.g., antacids) can temporarily affect thyroid hormone levels. Avoiding these for a few days before testing can help ensure accurate 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