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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 (free) thyroid hormone T4 in your blood, independent of protein binding.

Quick summary

["Free T4 helps assess your thyroid function and hormone balance."]

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

Key facts at a glance

LOINC
83121-4
Short code
T4 Free SerPl IA-sCnc
Synonyms
T4 Free SerPl IA-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
Method
ia
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 doctor may order a free T4 test if you have symptoms of thyroid dysfunction, such as fatigue, weight changes, or unexplained hair loss. It’s also used to monitor thyroid hormone levels in conditions like hypothyroidism or hyperthyroidism, especially when thyroid-stimulating hormone (TSH) test results are unclear.

How Is the Sample Collected?

A blood sample is taken from a vein in your arm. You may feel a small pinch when the needle goes in, but it usually doesn’t cause discomfort afterward. The sample is sent to a lab for testing.

What Are Common Causes of Abnormal Results?

Low free T4 levels may indicate hypothyroidism (underactive thyroid), while high levels could suggest hyperthyroidism (overactive thyroid) or certain medications affecting thyroid hormone metabolism. Other factors, like pregnancy or liver disease, can also influence results.

What Should I Expect Before or After the Test?

No special preparation is needed, but avoid smoking or drinking alcohol for 24 hours before your test. If you’re on thyroid medication, check with your doctor first. Results may take a few days to a week to receive.

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 does a free T4 test tell me about my thyroid health?
A free T4 test provides a direct measure of the biologically active form of thyroid hormone in your blood. Unlike total T4, which includes protein-bound hormone, free T4 reflects the portion that can cross cell membranes to exert its effects. It’s particularly useful for diagnosing thyroid disorders when TSH levels are normal or borderline, as free T4 helps identify subtle imbalances in thyroid function.
How does free T4 differ from total T4?
Free T4 is the unbound, or active, form of thyroid hormone, while total T4 includes both free and protein-bound hormone. Free T4 is often preferred because it’s less affected by changes in thyroid-binding proteins, which can occur with age, pregnancy, or certain medications.
Can lifestyle factors affect my free T4 test results?
While lifestyle factors like diet, stress, or sleep can influence thyroid hormone levels indirectly, they don’t directly alter the free T4 measurement itself. However, maintaining a healthy lifestyle may support overall thyroid function and help prevent thyroid-related symptoms.
  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