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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.

Quick summary

["Free T4 helps assess thyroid function and hormone balance."]

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

Key facts at a glance

LOINC
58838-4
Short code
T4 Free BS SerPl-mCnc
Synonyms
T4 Free BS 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 evaluate thyroid function, especially if you have symptoms like fatigue, weight changes, or unexplained hair loss. It’s often used alongside thyroid-stimulating hormone (TSH) tests to diagnose or monitor conditions such as hypothyroidism or hyperthyroidism.

How Is the Test Performed?

A blood sample is taken from a vein in your arm. The sample is then sent to a lab for analysis using an immunoassay method to measure the free T4 levels in your serum or plasma.

What Does the Test Result Mean?

Results can vary based on individual health, but they help determine whether your thyroid is producing too much or too little hormone. Abnormal results may prompt further testing or adjustments to medication.

Common Conditions Monitored

Free T4 is commonly used to diagnose and monitor conditions like hypothyroidism (underactive thyroid), hyperthyroidism (overactive thyroid), and thyroid nodules. It can also help assess thyroid hormone replacement therapy effectiveness.

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?
This test measures the concentration of free (unbound) thyroxine, a hormone that circulates freely in your blood and is directly available for your body’s use. Free T4 helps assess thyroid function independently of protein binding.
Why might my doctor order a free T4 test?
Your doctor may order a free T4 test if you have symptoms of thyroid issues (like fatigue, weight changes, or mood swings), if your TSH test results are unclear, or if you’re being monitored for thyroid-related conditions. It helps ensure accurate diagnosis and treatment.
Can free T4 levels change with diet or stress?
While diet and stress can influence thyroid function indirectly, free T4 levels are primarily measured in blood and are not directly affected by short-term dietary changes or stress. However, chronic stress or certain medical conditions may impact thyroid hormone regulation over time.
  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