Thyroid hormone total T4 (serum/plasma) measured by chemiluminescent immunoassay
This test measures the total amount of thyroxine (T4), a key hormone produced by your thyroid gland, in your blood. It helps assess thyroid function and whether your thyroid is working normally.
["Total T4 helps diagnose and monitor thyroid disorders like hypothyroidism or hyperthyroidism."]
Key facts at a glance
- LOINC
- 12923-9
- Short code
- T4 sp2 p chal SerPl-mCnc
- Synonyms
- T4 sp2 p chal 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 Is This Test Done?
Your healthcare provider may order this test if you have symptoms of thyroid problems, such as fatigue, weight changes, or unexplained mood swings. It may also be part of routine thyroid function testing, especially if you have a family history of thyroid disease or take medications that affect thyroid function.
What to Expect Before the Test
No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the test if your blood is drawn early in the morning. Avoid taking thyroid medications for 24 hours before the test if prescribed. Inform your provider about any medications or supplements you’re taking.
What Happens During the Test?
A healthcare professional will draw blood from a vein in your arm. The blood sample is then sent to a lab for analysis using a chemiluminescent immunoassay method, which detects the total T4 hormone levels in your serum or plasma.
What Do the Results Mean?
Results are typically reported as a ratio or concentration of T4 relative to a standard. While reference ranges vary by lab, abnormal results may indicate either low (hypothyroidism) or high (hyperthyroidism) thyroid hormone levels. Your provider will interpret results in the context of your symptoms and other tests.
Reference range
Adult: 4.5-12.0 μg/dLN/A
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 (Thyroid Hormone)
Frequently asked questions
How long does it take to get my results?
Can I eat or drink before this test?
Does this test require fasting?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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