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Free thyroxine (T4) concentration in serum/plasma

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

This test measures the amount of active thyroid hormone (T4) that is not bound to proteins in your blood, which can help assess your thyroid function and hormone balance.

Quick summary

["Free T4 helps doctors evaluate thyroid health by checking for overactive or underactive thyroid conditions."]

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

Key facts at a glance

LOINC
58840-0
Short code
T3Free BS SerPl-mCnc
Synonyms
T3Free BS SerPl-mCnc
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 Is This Test Done?

Your healthcare provider may order a free T4 test if you have symptoms like fatigue, weight changes, or unexplained hair loss, or if you have a known thyroid disorder. It’s also used alongside other thyroid tests to monitor treatment for conditions like hypothyroidism or hyperthyroidism.

How Is the Sample Collected?

A small amount of blood is drawn from a vein, usually in your arm, by a healthcare professional. The blood is then sent to a lab for testing.

What Does the Test Measure?

Free T4 measures the unbound (or ‘free’) form of T4, which is the active form of thyroid hormone that circulates in your blood and affects metabolism, growth, and other bodily functions.

What Are Common Reasons for Testing?

Tests for free T4 are often done when evaluating thyroid function, diagnosing thyroid disorders, or monitoring treatment for conditions like Graves’ disease, Hashimoto’s thyroiditis, or thyroid nodules.

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 free T4 test tell me about my thyroid health?
It provides a direct measure of your active thyroid hormone levels, helping doctors determine if your thyroid is producing too much or too little hormone. This can guide decisions about further testing or treatment.
How often should I get a free T4 test?
The frequency depends on your medical condition and treatment plan. Your healthcare provider will let you know how often to get tested, especially if you have an existing thyroid disorder or are taking thyroid medication.
Can lifestyle factors, like diet or stress, affect my free T4 levels?
Yes, certain foods (e.g., soy products), medications (e.g., certain blood pressure drugs), or stress can influence thyroid hormone levels. Always discuss your lifestyle and medications with your doctor before testing.
  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