Thyroid Hormone, Total (Serum or Plasma)
This test measures the total amount of thyroxine (T4), a key hormone produced by your thyroid gland. It helps assess thyroid function and whether your body is making enough thyroid hormone.
["Total T4 helps diagnose and monitor thyroid disorders like hypothyroidism or hyperthyroidism."]
Key facts at a glance
- LOINC
- 12924-7
- Short code
- T4 sp3 p chal SerPl-mCnc
- Synonyms
- T4 sp3 p chal 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 Is This Test Done?
Your healthcare provider may order a total T4 test to check for thyroid issues, such as underactive (hypothyroidism) or overactive (hyperthyroidism) thyroid function. It can also be used during pregnancy, before starting thyroid hormone treatment, or if you have symptoms like fatigue, weight changes, or unexplained mood swings.
What Does the Test Measure?
Total T4 includes both active and inactive forms of thyroxine in your blood. This measurement helps determine whether your thyroid gland is functioning normally or if it’s producing too much or too little hormone.
Sample Collection and Preparation
A blood sample is taken from a vein in your arm. The blood is collected in a special tube and sent to a lab for analysis. You may need to fast for a few hours before the test, but your provider will let you know if fasting is required.
What to Expect Before the Test
There are usually no special preparations needed, but avoid smoking or drinking alcohol before the test. If you’re on thyroid medication, ask your provider whether to stop temporarily or keep taking it as usual.
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
A total T4 test is often part of a broader thyroid function evaluation, sometimes alongside TSH (thyroid-stimulating hormone) tests. Low total T4 levels may indicate hypothyroidism, while high levels could suggest hyperthyroidism or other conditions like thyroiditis. Results help guide treatment decisions, such as adjusting thyroid hormone medication.
If your total T4 is low, your doctor may recommend additional tests (like TSH) or adjust your thyroid medication. Symptoms like fatigue, weight gain, or cold intolerance may also prompt further evaluation.
Check with your provider about fasting rules, but generally, you can eat and drink normally unless instructed otherwise. Avoid smoking or heavy alcohol consumption before the test.
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