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Thyroid hormone total serum concentration (T4)

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

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

Quick summary

["Your doctor may check T4 levels to diagnose or monitor thyroid disorders like hypothyroidism or hyperthyroidism."]

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

Key facts at a glance

LOINC
14921-1
Short code
T4 SerPl-sCnc
Synonyms
T4 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 This Test Is Done

Your healthcare provider may order a T4 test if you have symptoms like fatigue, weight changes, or unexplained mood swings. It’s also used to check thyroid function before or after treatment for thyroid conditions.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the blood draw if your test is part of a comprehensive thyroid panel.

How the Test Works

A small amount of blood is drawn from a vein in your arm. The sample is sent to a lab where it’s analyzed to measure T4 levels.

What the Results Mean

Results help determine if your thyroid is working normally or if it’s overactive (hyperthyroidism) or underactive (hypothyroidism). Your doctor will compare your results to your symptoms 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.

Clinical Significance: Thyroid Hormone Imbalance

Short name

Total Serum Thyroid Hormone (T4)

Frequently asked questions

What could cause abnormal T4 results?
Abnormal T4 levels can be caused by thyroid diseases (e.g., Hashimoto’s disease, Graves’ disease), thyroid nodules, pituitary disorders, or medications that affect thyroid function. High or low levels may also indicate other health issues.
How often should I get this test?
Your doctor will decide how often to check your T4 levels based on your medical history, symptoms, and treatment plan. Regular monitoring may be needed if you have a thyroid condition or take thyroid medication.
Can diet affect T4 test results?
Yes, certain foods (like soy, cruciferous vegetables) or supplements (e.g., selenium, iodine) may influence thyroid function. Avoiding these for a few days before testing can help ensure accurate results.
  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