Thyroid hormone total serum concentration (mass concentration)
This test measures the total amount of thyroxine (T4), a key hormone produced by your thyroid gland. It helps regulate metabolism, energy levels, and other bodily functions.
["Your doctor may check T4 levels to diagnose or monitor thyroid conditions like hypothyroidism or hyperthyroidism."]
Key facts at a glance
- LOINC
- 3026-2
- Short code
- T4 SerPl-mCnc
- Synonyms
- T4 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 Matters for Your Health
Thyroid hormones like T4 play a vital role in controlling how your body uses energy. Low or high levels can lead to symptoms like fatigue, weight changes, or heart issues. This test helps doctors track thyroid function and guide treatment plans.
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 draw if your lab uses a specific method. Wear a comfortable shirt for easy access to your arm.
How the Test Works
A healthcare professional will draw blood from a vein in your arm. The sample is sent to a lab where it’s analyzed to measure the total T4 concentration in your serum.
Common Reasons for This Test
Doctors order this test to evaluate thyroid function, check for thyroid disorders, monitor treatment for conditions like hypothyroidism or hyperthyroidism, or assess symptoms such as fatigue, weight changes, or heart palpitations.
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.
Abnormally low T4 levels may indicate hypothyroidism (underactive thyroid), which can cause fatigue, weight gain, and depression. High levels may suggest hyperthyroidism (overactive thyroid), leading to weight loss, anxiety, or rapid heartbeat. Results help guide treatment, such as hormone replacement or medication adjustments.
Short name
Total Serum T4
Frequently asked questions
What Does a Low or High T4 Level Mean?
Do I Need to Fast Before This Test?
How Long Does It Take to Get 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