Thyroid hormone total (T4) measured by mass concentration in serum/plasma
This test checks the amount of total thyroid hormone (T4) in your blood, which helps evaluate thyroid function.
["Total T4 levels can indicate whether your thyroid is producing enough hormone or if there’s an imbalance."]
Key facts at a glance
- LOINC
- 3020-5
- Short code
- T4BG Bld-mCnc
- Synonyms
- T4BG Bld-mCnc
- Category
- chemistry
- Sample
- whole-blood
- Container
- Lavender-top (EDTA)
- 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
- 3 mL
- Stability
- Room temperature: 4h, Refrigerated: 24h
- Hemolysis max
- 0
- Lipemia max
- 0
- Icterus max
- 0
- TAT
- 4–24 hours
- Order
- routine
Why This Test Is Done
Your healthcare provider may order this test to diagnose or monitor conditions like hypothyroidism (underactive thyroid), hyperthyroidism (overactive thyroid), or thyroid hormone replacement therapy. It can also help assess thyroid function before or after treatment for thyroid disorders.
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 it’s drawn early in the morning. Avoid smoking or consuming caffeine for a few hours before testing. If you’re on thyroid medication, check with your provider first to confirm whether to stop temporarily.
What the Test Results Mean
Results are compared to a general reference range, but individual ranges can vary by lab. Low levels may suggest hypothyroidism, while high levels could indicate hyperthyroidism or other conditions. Your provider will interpret results in the context of your symptoms and medical history.
Common Conditions This Test Helps Diagnose
This test is often used alongside other thyroid tests (like TSH) to confirm or rule out thyroid disorders. It can also help monitor thyroid function during pregnancy or in people with autoimmune thyroid diseases.
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 levels provide a snapshot of your body’s active thyroid hormone supply. While TSH (thyroid-stimulating hormone) is often the first test for thyroid issues, T4 helps confirm whether your thyroid is producing enough hormone to meet your body’s needs. Abnormal results may prompt further testing or adjustments to treatment.
Short name
Total T4 (Serum/Plasma)
Frequently asked questions
Will I need to avoid certain foods or drinks before this test?
Can I take my thyroid medication before or after this test?
What if my T4 level is 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