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Free thyroxine (T4) concentration in serum/plasma

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

This test measures the amount of free (unbound) thyroxine, 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

["Free T4 helps assess your thyroid function and whether your thyroid is producing enough hormone."]

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

Key facts at a glance

LOINC
25797-2
Short code
T4 Free sp8 p chal SerPl-sCnc
Synonyms
T4 Free sp8 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 if you have symptoms of an underactive thyroid (hypothyroidism), such as fatigue, weight gain, or cold sensitivity, or if you have an overactive thyroid (hyperthyroidism), like rapid heartbeat or anxiety. It may also be used to monitor thyroid hormone levels during treatment for thyroid disorders.

What to Expect Before the Test

No special preparation is needed for this test. You may have a blood sample drawn from a vein in your arm, similar to other blood tests. If you’re on thyroid hormone medication, your provider may ask you to stop it temporarily before the test.

What the Results Mean

Free T4 levels can vary based on individual health, diet, and other factors. Your results will be compared to a reference range provided by your lab. Low levels may indicate hypothyroidism, while high levels may suggest hyperthyroidism or other conditions.

Common Causes of Abnormal Results

Abnormal free T4 levels can be caused by thyroid disorders (such as Hashimoto’s disease or Graves’ disease), thyroid nodules, or other conditions affecting thyroid hormone production or metabolism. Stress, pregnancy, or certain medications can also influence 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 does a low free T4 level mean?
A low free T4 level often suggests hypothyroidism, meaning your thyroid isn’t producing enough hormone. This can lead to symptoms like fatigue, weight gain, or depression. Your provider may recommend further tests, such as thyroid-stimulating hormone (TSH) levels, to diagnose the cause and guide treatment.
What does a high free T4 level mean?
A high free T4 level may indicate hyperthyroidism, where your thyroid is overactive. This can cause symptoms like rapid heartbeat, weight loss, or anxiety. Additional tests, such as TSH or other thyroid antibodies, may be needed to determine the exact cause and treatment plan.
Can diet affect free T4 test results?
Yes, certain foods and supplements, like soy, selenium, or iodine-rich foods, can temporarily affect thyroid hormone levels. If you’re taking thyroid medication, avoid consuming large amounts of these foods before your test to get more accurate results. Always follow your provider’s instructions.
  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