Total thyroxine (T4) concentration in serum
This test measures the amount of thyroxine (T4), a hormone produced by your thyroid gland, in your blood serum. It helps assess thyroid function and identify potential thyroid disorders.
["A T4 test helps check if your thyroid is working properly and may be used alongside other thyroid tests."]
Key facts at a glance
- LOINC
- 38356-2
- Short code
- T4 Ab Ser Ql
- Synonyms
- T4 Ab Ser Ql
- 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 key hormone produced by your thyroid gland. It regulates metabolism, energy levels, and many other bodily functions. Your body converts T4 into an active form called triiodothyronine (T3) for use throughout your body. This test specifically measures the total amount of T4 in your blood.
Why Might You Need This Test?
Doctors may order a T4 test to diagnose or monitor conditions such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also be used during pregnancy, before surgery, or if you have symptoms like fatigue, weight changes, or unexplained mood swings.
Common Uses and Indications
A T4 test is often part of a thyroid function panel, which may include other tests like thyroid-stimulating hormone (TSH). It helps confirm or clarify thyroid hormone levels when symptoms suggest an issue with your thyroid.
What to Expect Before the Test
No special preparation is usually needed for this test. You may be asked to fast for 8–12 hours before the blood draw, but this depends on your healthcare provider’s instructions. Bring any medications you’re currently taking to your appointment.
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.
◆Associated findings1
Short name
Total T4
Frequently asked questions
How long will it take to get my results?
Can I take my thyroid medication before the test?
What if my T4 levels are outside of normal ranges?
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