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Thyroid Hormone, Free T4 (Serum)

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

This test measures the amount of free thyroxine (T4), a hormone produced by your thyroid gland that helps regulate metabolism and energy levels.

Quick summary

["Free T4 helps assess thyroid function and detect potential thyroid disorders."]

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

Key facts at a glance

LOINC
12928-8
Short code
T4 sp7 p chal SerPl-mCnc
Synonyms
T4 sp7 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 healthcare provider may order a free T4 test to check for thyroid problems, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also be used to monitor thyroid hormone levels during treatment for thyroid conditions.

What Does a Free T4 Test Involve?

A blood sample is taken from a vein, usually in your arm. The sample is then sent to a lab for analysis. No special preparation is typically needed, but you may be asked to fast for a few hours before the test if instructed by your provider.

What Are the Possible Results?

Results are interpreted by your healthcare provider based on your medical history, symptoms, and other test results. Abnormal levels may indicate thyroid dysfunction, which can affect how your body uses energy and other vital functions.

What Are Common Causes of Abnormal Results?

Low free T4 levels may suggest hypothyroidism, while high levels could indicate hyperthyroidism. Other conditions, such as thyroid nodules, pituitary disorders, or certain medications, may 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 free T4 test measure exactly?
It measures the amount of free (unbound) thyroxine hormone in your blood, which is not attached to proteins and can directly affect your body’s metabolism and functions.
Can diet affect my free T4 test results?
Some foods and medications can temporarily affect thyroid hormone levels, but your provider may give specific instructions. Generally, avoid certain supplements or foods (like soy) for a few days before testing if advised.
How often should I get a free T4 test?
Frequency depends on your health condition and treatment. Your healthcare provider will determine how often you need testing, whether it’s annually or more frequently if you have thyroid issues or are under treatment.
  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