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Thyroid hormone total (T4) in serum, measured by immunoassay

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

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

Quick summary

["Your doctor may order this test to check thyroid function or monitor thyroid hormone levels."]

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

Key facts at a glance

LOINC
3021-3
Short code
T4BG SerPl-mCnc
Synonyms
T4BG SerPl-mCnc
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

Doctors may request a total T4 test for several reasons, including diagnosing or managing conditions like hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also be used to screen for thyroid issues or monitor treatment progress for conditions like Graves’ disease or thyroid nodules.

What to Expect Before the Test

No special preparation is needed for this blood test. You may be asked to fast for 8–12 hours before the draw if your doctor specifies it, but this is not always required for T4 testing alone.

What the Test Involves

A healthcare professional will draw blood from a vein in your arm. You may feel a slight pinch, but the process is quick and usually painless. The blood sample will be sent to a lab for analysis.

What the Results Mean

Results are interpreted in relation to your overall health, symptoms, and other lab tests. Abnormally low or high T4 levels may signal thyroid dysfunction, but they can also be influenced by other factors like medications or stress.

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

Total T4

Frequently asked questions

What Are Common Causes of Abnormal Results?
Low T4 levels may indicate hypothyroidism, thyroiditis, or conditions affecting thyroid hormone production. High T4 levels can occur in hyperthyroidism, thyroid nodules, or rare metabolic disorders. Always discuss results with your doctor, as they’ll consider your full medical picture.
How Often Should I Get This Test?
Your doctor will determine how often you need retesting based on your symptoms, medical history, and treatment plan. Regular monitoring may be needed if you have a thyroid disorder or take thyroid medication.
Can I Eat or Drink Before the Test?
Most of the time, no dietary restrictions apply to T4 testing alone. However, if your doctor has ordered additional tests (like TSH), they may specify fasting. Always follow their 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