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Free thyroxine (T4) concentration in serum or plasma

Published: August 9, 2026Updated: August 17, 2026

This test measures the amount of free thyroxine (T4), a hormone produced by your thyroid gland that travels freely in your bloodstream. Free T4 is important because it reflects your active thyroid hormone levels.

Quick summary

["Free T4 helps doctors assess your thyroid function and whether your thyroid is producing enough hormone for your body’s needs."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
65351-9
Short code
T3Free pre/p chal SerPl-sCnc
Synonyms
T3Free pre/p chal SerPl-sCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
80
Ref high
200
Critical range
20 N/A – 400 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84436
SNOMED
302346007
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 This Test Is Done

Your healthcare provider may order a free T4 test if you have symptoms of an underactive thyroid (hypothyroidism), such as fatigue, weight changes, 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 usually needed. You may be asked to fast for 8–12 hours before the test if your blood is drawn early in the morning. Avoid smoking or drinking alcohol for a few hours before the test. If you’re on thyroid medication, check with your provider whether to stop temporarily.

How the Test Is Done

A healthcare professional will draw blood from a vein in your arm using a small needle. You may feel a brief pinch, but the procedure is quick. The blood sample is then sent to a lab for analysis.

What the Results Mean

Free T4 levels alone don’t diagnose thyroid disorders—they’re often used alongside other tests like thyroid-stimulating hormone (TSH). Low free T4 may indicate hypothyroidism, while high levels may suggest hyperthyroidism or other conditions affecting thyroid hormone metabolism.

Reference range

Adult: 80-200 ng/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 high free T4 level mean?
High free T4 can signal hyperthyroidism (overactive thyroid) or conditions like thyroiditis (inflammation of the thyroid) or certain medications that interfere with hormone binding. It may also occur in rare cases of thyroid hormone resistance. Always discuss results with your provider, as they’ll consider your full medical picture.
What does a low free T4 level mean?
Low free T4 often points to hypothyroidism, where the thyroid isn’t producing enough hormone. It can also occur in pregnancy, certain medications (like some antidepressants or lithium), or during stress. Your provider may recommend further testing to identify the cause.
Can free T4 levels change with diet?
Yes, certain foods and supplements can influence thyroid hormone levels. For example, high iodine intake (from seafood, supplements, or iodized salt) may temporarily affect thyroid function. Avoiding these before your test can help ensure accurate results. Always follow your provider’s guidance.
  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 17, 2026. · Read our editorial policy