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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, unbound thyroid hormone (T4) in your blood, which helps assess your thyroid function and whether your thyroid is producing enough hormone.

Quick summary

["Free T4 helps 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
25794-9
Short code
T4 Free sp5 p chal SerPl-sCnc
Synonyms
T4 Free sp5 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 It’s Done

Your healthcare provider may order this test if you have symptoms of thyroid problems, such as unexplained weight changes, fatigue, or changes in your menstrual cycle. It’s also used to check if thyroid hormone replacement therapy is working properly or if you need adjustments.

What to Expect

A small 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 testing.

How It Works

The test measures the free (unbound) form of T4, which is the biologically active form of thyroid hormone. Unlike total T4, free T4 accounts for hormone levels that actually reach your tissues, making it a more accurate measure of thyroid function.

What the Results Mean

Results are not provided as a specific range here, as reference intervals vary by lab and patient factors. Your healthcare provider will interpret the results in relation to your symptoms and other test results.

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 free T4 levels to change?
Free T4 levels can be affected by thyroid diseases (like Graves’ disease or Hashimoto’s thyroiditis), thyroid hormone medications, pregnancy, certain medications (e.g., steroids, amiodarone), or conditions that alter protein binding in the blood.
How often should I get this test?
Your provider will determine how often you need this test based on your medical history, symptoms, and treatment plan. Regular monitoring may be needed if you have a thyroid disorder or take thyroid medication.
Can diet or lifestyle affect free T4 levels?
Dietary changes, such as consuming high amounts of iodine (e.g., seafood, supplements) or selenium, can temporarily affect thyroid hormone levels. Stress, sleep, and certain medications may also influence results, so it’s best to discuss these factors with your 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