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Total thyroxine (T4) concentration in serum/plasma

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

This test measures the amount of thyroxine (T4), a hormone produced by your thyroid gland that helps regulate metabolism.

Quick summary

["T4 levels help assess thyroid function and identify potential thyroid disorders."]

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

Key facts at a glance

LOINC
14123-4
Short code
T4 Alb Bnd/Alb SerPl
Synonyms
T4 Alb Bnd/Alb SerPl
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 T4 test to check for thyroid problems, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also be used to monitor thyroid hormone levels during treatment for thyroid conditions.

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 your blood is drawn early in the morning, as T4 levels can vary slightly with timing.

How the Test Is Performed

A healthcare professional will draw blood from a vein in your arm using a small needle. You may feel a slight pinch, but it usually doesn’t cause pain. The blood sample is sent to a lab for analysis.

What the Results Mean

T4 results alone may not always indicate a thyroid disorder, as other thyroid hormones (like T3 or thyroid-stimulating hormone, TSH) also play a role. Your results will be reviewed in the context of your overall health and other tests.

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.

Elevated T4 levels may suggest hyperthyroidism or other conditions affecting thyroid hormone metabolism. Low T4 levels can indicate hypothyroidism or conditions like pregnancy, stress, or certain medications interfering with hormone production. Always discuss results with your provider, as they consider your full medical picture.

Short name

T4 Serum/Plasma

Frequently asked questions

What causes T4 levels to be abnormal?
Abnormal T4 levels can result from thyroid disorders (e.g., Graves’ disease, Hashimoto’s thyroiditis), thyroid removal or treatment (e.g., radioactive iodine), pregnancy, or certain medications (e.g., amiodarone, steroids). Other conditions like kidney disease or liver problems may also influence results.
How often should I get this test?
Your provider will determine how often you need T4 testing based on your symptoms, medical history, and treatment plan. Some people with stable thyroid conditions may need testing every few months or annually, while others may need more frequent checks.
Can diet affect T4 test results?
Yes, certain foods and supplements (e.g., soy, selenium, iodine-rich foods) may temporarily affect thyroid hormone levels. Avoiding large amounts of these for a few days before your test can help ensure more accurate results. Always follow your provider’s specific instructions.
  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