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Serum or Plasma Free Thyroxine (T4) Concentration

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

This test measures the amount of free thyroxine (T4), the active form of thyroid hormone circulating in your blood. Free T4 helps regulate metabolism, growth, and development.

Quick summary

["Free T4 reflects your thyroid’s active hormone levels, guiding treatment for thyroid disorders."]

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

Key facts at a glance

LOINC
77460-4
Short code
Deprecated Hind+Mid ft XR
Synonyms
Deprecated Hind+Mid ft XR
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
xr
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 It’s Done

Your healthcare provider may order a free T4 test to diagnose or monitor thyroid conditions, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It’s often used alongside thyroid-stimulating hormone (TSH) tests for a complete picture.

What to Expect

A blood sample is taken from a vein in your arm. You may feel a slight pinch, but it’s usually quick. No special preparation is needed unless instructed otherwise.

How Results Are Used

Results help determine if your thyroid is functioning normally. Abnormal levels may indicate the need for medication adjustments, further testing, or treatment.

Common Conditions Monitored

Free T4 is used to assess thyroid hormone balance in conditions like Graves’ disease, Hashimoto’s thyroiditis, or after treatment for thyroid disorders.

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?
It measures the concentration of free thyroxine (T4) in your blood, which is the hormone that actively influences metabolism and other bodily functions.
How often should I get a free T4 test?
Your provider will determine how often based on your medical history, symptoms, and other test results. Follow their guidance for timing and frequency.
Can diet or supplements affect free T4 test results?
Yes, certain foods (like soy products), medications (e.g., birth control, antacids), or supplements (e.g., selenium) may temporarily affect thyroid hormone levels. Avoid these for 2–4 weeks before testing if instructed.
  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