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Serum/Plasma Free Thyroxine (T4) Concentration

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 your thyroid function and whether your thyroid is producing enough hormone.

Quick summary

["Free T4 levels help diagnose and monitor thyroid disorders like hypothyroidism or hyperthyroidism."]

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

Key facts at a glance

LOINC
57507-6
Short code
11DC 45M p 250 ug ACTH SerPl-mCnc
Synonyms
11DC 45M p 250 ug ACTH SerPl-mCnc
Category
hormone
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
10
Ref high
60
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 This Test Matters

Your thyroid produces hormones that regulate metabolism, energy, and many other bodily functions. Free T4 is the form of thyroid hormone that directly affects cells in your body. Measuring it helps doctors determine if your thyroid is working properly or if you need treatment.

What to Expect Before the Test

No special preparation is usually needed. You may be asked to fast for 8–12 hours before the test if it involves drawing blood from your arm. Wear comfortable clothing and bring any medications you’re currently taking.

How the Test Works

A healthcare provider will draw blood from a vein in your arm. The blood sample is then sent to a lab where it is analyzed to measure the concentration of free T4. The test is typically done using a sensitive immunoassay method to detect small amounts of the hormone.

What the Results Mean

Results are compared to a reference range that varies by lab. High or low levels may indicate thyroid dysfunction, which can cause symptoms like fatigue, weight changes, or mood swings. Your doctor will interpret the results in the context of your symptoms and other tests.

Reference range

NCBI: ACTH 10-60 pg/mLN/A

Specimen: serum

Typically collected in the morning (8-10 AM). Inform your provider if taking steroid medications.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Free T4

Frequently asked questions

Why might my doctor order this test?
Your doctor may recommend a free T4 test if you have symptoms of thyroid problems (e.g., unexplained weight changes, fatigue, or hair loss), if you have a family history of thyroid disease, or if you have other thyroid-related tests (like TSH) that show abnormal results. It’s also used to monitor thyroid hormone treatment.
What do my results mean if they’re outside the normal range?
If your free T4 is low, it may suggest hypothyroidism (underactive thyroid), while high levels could indicate hyperthyroidism (overactive thyroid). However, results alone don’t diagnose a condition—they guide further evaluation, such as additional thyroid tests or a physical exam.
How often should I get this test?
The frequency depends on your health status. If you have a stable thyroid condition, your doctor may recommend testing every 6–12 months. If you’re newly diagnosed or undergoing treatment, testing may be more frequent to monitor progress.
  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