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Free thyroxine (T4) concentration in serum

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

This test measures the amount of free thyroxine (T4), the active form of thyroid hormone circulating in your blood. It helps assess how well your thyroid is functioning and whether your medication or condition requires adjustment.

Quick summary

["Free T4 levels help diagnose and monitor thyroid disorders and thyroid hormone treatment."]

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

Key facts at a glance

LOINC
98048-2
Short code
Levothyroxine IgE Qn
Synonyms
Levothyroxine IgE Qn
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
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 issues with metabolism. It’s often used alongside thyroid-stimulating hormone (TSH) tests to confirm or clarify thyroid hormone levels.

What to Expect Before the Test

No special preparation is needed for this test. You’ll have a small needle inserted into your arm to draw blood. It usually takes a few minutes, and you can resume normal activities afterward. Avoid smoking or drinking coffee immediately before the test if advised by your provider.

What the Results Mean

Free T4 results help determine if your thyroid is producing enough active hormone. Abnormal levels may indicate conditions like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), which can affect energy, weight, and metabolism.

Common Conditions This Test Helps Diagnose

This test is commonly used to diagnose or monitor thyroid disorders, such as Hashimoto’s thyroiditis, Graves’ disease, or thyroid nodules. It’s also useful in evaluating thyroid hormone replacement therapy effectiveness.

Reference range

NCBI: T4 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 Diagnosing and Managing Thyroid Disorders

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 that isn’t bound to proteins in your blood. Total T4 includes both free and bound T4, so free T4 provides a clearer picture of your body’s active hormone levels. If your provider orders both, they may compare the results to better assess thyroid function.
Can fasting affect free T4 test results?
Generally, fasting is not required for this test, but some providers may advise against eating or drinking certain substances (like caffeine) before the draw to avoid potential interference. Always follow your healthcare provider’s specific instructions.
What if my free T4 level is outside the reference range?
If your free T4 level is low, it may indicate hypothyroidism, while high levels could suggest hyperthyroidism. Your provider will discuss these results with you and may recommend additional tests or adjustments to your treatment plan, such as medication or lifestyle changes.
  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