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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 (free) thyroid hormone T4 in your blood, which is not bound to proteins. Free T4 is crucial for regulating metabolism and energy levels.

Quick summary

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

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

Key facts at a glance

LOINC
58836-8
Short code
T4 Free 3h p chal SerPl-mCnc
Synonyms
T4 Free 3h p chal SerPl-mCnc
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 doctor may order a free T4 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 monitor thyroid hormone levels during treatment for conditions like hypothyroidism or hyperthyroidism.

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 test if your blood is drawn early in the morning. Wear comfortable clothing and plan to stay near the lab for about 15 minutes while the sample is drawn.

What Happens During the Test?

A healthcare provider will insert a small needle into your arm to draw blood. You may feel a slight pinch, but the procedure takes only a few minutes. The blood sample is then sent to a lab for analysis.

What Are Possible Results?

Results are typically reported as a concentration value (in nanograms per deciliter, or ng/dL). Your doctor will compare your result to your individual reference range, which may vary slightly depending on your age, sex, and overall health.

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.

Clinical Significance: Free T4’s Role in Health

Short name

Free T4

Frequently asked questions

How does free T4 differ from total T4?
Free T4 is the biologically active form of thyroid hormone, while total T4 includes both free and protein-bound T4. Protein-bound T4 isn’t directly available to your body’s tissues, so free T4 is the more relevant measure for assessing thyroid function and potential hormone imbalances.
What symptoms might indicate abnormal free T4 levels?
Low free T4 may cause symptoms like fatigue, weight gain, cold intolerance, dry skin, or depression. High free T4 can lead to symptoms such as anxiety, weight loss, rapid heartbeat, or heat intolerance. If you experience these symptoms, discuss them with your doctor.
Can diet or medications affect free T4 test results?
Yes, certain medications (like some blood pressure or thyroid medications) and dietary supplements (e.g., soy, selenium) can influence thyroid hormone levels. Your doctor may advise you to avoid these temporarily or adjust your treatment plan based on the test 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