Thyroid hormone total serum concentration (T4)
This test measures the amount of the primary thyroid hormone, thyroxine (T4), in your blood serum. T4 helps regulate metabolism, energy levels, and growth in your body.
["Your T4 levels help doctors assess thyroid function and identify potential thyroid disorders."]
Key facts at a glance
- LOINC
- 27980-2
- Short code
- T4BG SerPl-sCnc
- Synonyms
- T4BG 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 Is This Test Done?
Doctors may order a T4 test to check for thyroid problems, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also be used as part of a routine check-up or to monitor thyroid treatment effectiveness.
What Does a T4 Test Measure?
The test measures the total amount of T4 in your blood, including both bound and free T4. Free T4 is the active form that actually works in your body, but the total T4 level is often used as a general indicator of thyroid function.
Sample Preparation and Collection
A blood sample is collected from a vein in your arm. You may feel a slight pinch when the needle is inserted, but the procedure is quick and usually painless. No special preparation is needed, but fasting may be required depending on the clinic’s instructions.
What to Expect Before the Test
You may be asked to avoid certain medications or foods that could affect results, such as iodine supplements or certain medications. If you have any concerns, discuss them with your healthcare provider before the test.
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.
Abnormal T4 levels can indicate underlying thyroid conditions. Low T4 levels may suggest hypothyroidism, which can cause fatigue, weight gain, and cold intolerance. High T4 levels may suggest hyperthyroidism, leading to weight loss, anxiety, and rapid heartbeat. However, T4 alone is not always definitive; other thyroid tests (like TSH) may be needed for diagnosis.
Short name
Total Serum Thyroid Hormone (T4)
Frequently asked questions
How long does it take to get my T4 test results?
Can I eat or drink before this test?
What does it mean if my T4 level is outside of the normal range?
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