Thyroid hormone total serum concentration (T4)
This test measures the amount of total thyroid hormone (T4) in your blood. T4 is a hormone produced by your thyroid gland that plays a key role in regulating metabolism.
["Your doctor may check your T4 levels to diagnose or monitor thyroid conditions like hypothyroidism or hyperthyroidism."]
Key facts at a glance
- LOINC
- 12926-2
- Short code
- T4 sp5 p chal SerPl-mCnc
- Synonyms
- T4 sp5 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 This Test Is Done
Doctors order total T4 tests to evaluate thyroid function. It’s often used as part of a broader thyroid panel that includes tests for thyroid-stimulating hormone (TSH) and sometimes free T4. This test helps identify whether your thyroid is producing too much or too little hormone, which can affect your energy, weight, and overall health.
What to Expect Before the Test
You won’t need special preparation for this blood test. On the day of your visit, avoid smoking, drinking alcohol, or eating a heavy meal. Your blood will be drawn from a vein, usually in your arm, and it typically takes a few minutes. You may feel a slight pinch or discomfort, but it usually goes away quickly.
What the Results Mean
Total T4 levels alone don’t always give a complete picture of thyroid health because they can be influenced by factors like medications, pregnancy, or liver disease. Your doctor will consider your T4 results alongside other tests (like TSH) and your symptoms to determine if treatment is needed.
Common Conditions Linked to T4 Levels
Abnormal T4 levels can signal conditions such as hypothyroidism (low T4), hyperthyroidism (high T4), or thyroiditis (inflammation of the thyroid). However, other health issues, like kidney or liver problems, can also affect T4 measurements.
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.
Total T4 is a key marker in diagnosing and managing thyroid disorders. Low levels may indicate hypothyroidism, while high levels can suggest hyperthyroidism or other thyroid dysfunctions. Your doctor will use these results to guide treatment, such as medication adjustments or further testing.
Short name
Total T4
Frequently asked questions
What causes my T4 levels to be out of range?
How often should I get this test?
Can diet or lifestyle affect my T4 test results?
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