Total thyroxine (T4) concentration in serum
This test measures the amount of thyroxine (T4), a key hormone produced by your thyroid gland, in your blood serum. T4 helps regulate metabolism and energy levels.
["Your thyroid hormone levels help control how your body uses energy."]
Key facts at a glance
- LOINC
- 9580-2
- Short code
- T4 Ab Ser-aCnc
- Synonyms
- T4 Ab Ser-aCnc
- 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
What Is Thyroxine (T4)?
Thyroxine (T4) is a hormone made by your thyroid gland. It plays a vital role in controlling metabolism, growth, and development. Your body converts T4 into another hormone called triiodothyronine (T3), which is more active in regulating energy and body functions. T4 levels can help doctors diagnose thyroid disorders, such as hypothyroidism or hyperthyroidism.
Why Might You Need This Test?
Doctors may order a T4 serum test if you have symptoms like fatigue, weight changes, or unexplained issues with metabolism. It’s also used to monitor thyroid conditions, such as Hashimoto’s disease or thyroid nodules, or to check for thyroid dysfunction before or after treatment.
When Is This Test Done?
This test is typically done as part of a thyroid function panel. You may need to fast for 8–12 hours before the blood draw, but this depends on your doctor’s instructions. Blood is taken from a vein in your arm, and the sample is sent to a lab for analysis.
What to Expect Before the Test
There are usually no special preparations needed, but your doctor may ask you to avoid certain medications or foods that could affect results. You may feel slight discomfort during the blood draw, but it usually doesn’t cause long-term effects.
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.
How Does This Test Help Your Doctor?
Short name
T4 Serum
Frequently asked questions
How long does it take to get my T4 test results?
Can diet or medications affect my T4 test results?
What if my T4 levels are outside 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