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Free thyroxine (T4) concentration in serum/plasma

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

This test measures the amount of active, unbound (free) thyroxine hormone in your blood, which is important for regulating metabolism and energy levels.

Quick summary

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

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

Key facts at a glance

LOINC
14920-3
Short code
T4 Free SerPl-sCnc
Synonyms
T4 Free SerPl-sCnc
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 health, especially if you have symptoms like fatigue, weight changes, or unexplained goiter. It’s often used alongside thyroid-stimulating hormone (TSH) testing to diagnose conditions such as hypothyroidism or hyperthyroidism.

How Is the Sample Collected?

A blood sample is drawn from a vein, usually in your arm. The blood is processed to separate serum or plasma for testing.

What Does a Free T4 Test Tell You?

Free T4 levels help determine whether your thyroid is producing enough active hormone. Low levels may indicate hypothyroidism, while high levels could suggest hyperthyroidism or other thyroid-related issues. It’s particularly useful when TSH results are unclear.

Possible Causes of Abnormal Results

Changes in free T4 can occur due to thyroid disorders (e.g., Hashimoto’s disease, Graves’ disease), medications affecting thyroid function, pregnancy, or kidney/liver disease. Stress, diet, or certain medications (like beta-blockers) 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 Are Common Symptoms to Watch For?
Symptoms like unexplained weight changes, fatigue, muscle weakness, or changes in mood may prompt a free T4 test. Always discuss symptoms with your provider, as they can guide further testing.
How Often Should This Test Be Done?
Frequency depends on your medical history and symptoms. Routine screening for thyroid disorders is not typically recommended unless you have risk factors (e.g., family history, autoimmune conditions). Follow your provider’s advice.
Can Diet or Medications Affect Free T4 Levels?
Yes. Certain foods (e.g., soy products), medications (e.g., thyroid hormone replacement, antithyroid drugs), and supplements can temporarily alter free T4 levels. Avoid dietary changes or supplements 2–4 weeks before testing if possible.
  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