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Thyroid hormone total serum concentration (T4)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of thyroxine (T4), a hormone produced by your thyroid gland that helps regulate metabolism.

Quick summary

["Your doctor may check T4 levels to diagnose or monitor thyroid disorders."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
83119-8
Short code
T4 SerPl IA-mCnc
Synonyms
T4 SerPl IA-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
Method
ia
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 Is Done

Your doctor might order a T4 test if you have symptoms of an overactive or underactive thyroid, such as unexplained weight changes, fatigue, or changes in heart rate. It may also be used during thyroid function evaluations, before or after thyroid treatments, or to check for thyroid hormone imbalances.

What to Expect Before the Test

No special preparation is usually needed. You may be asked to fast for 8–12 hours before the test if your blood is drawn in the morning, but this depends on your doctor’s instructions. Wear comfortable clothing and plan to stay near the testing site for a short time.

How the Test Is Done

A healthcare provider will insert a small needle into a vein in your arm to draw blood. You may feel a brief pinch, but it usually doesn’t cause discomfort. The blood sample is then sent to a lab for analysis.

What the Results Mean

Results help determine whether your thyroid is producing enough T4 or too little. Abnormal levels can indicate conditions like hypothyroidism (low T4) or hyperthyroidism (high T4), which may require further testing or treatment.

Reference range

Adult: 4.5-12.0 μg/dLN/A

Specimen: serum

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 Serum Thyroxine (T4)

Frequently asked questions

What causes T4 test results to be abnormal?
Abnormal T4 results can be caused by thyroid disorders (such as hypothyroidism or hyperthyroidism), thyroid nodules, or certain medications (e.g., antithyroid drugs). Other factors, like pregnancy, stress, or certain medical conditions, may also affect levels.
How long does it take to get T4 test results?
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your doctor will discuss the findings with you during a follow-up visit.
Can lifestyle changes affect T4 test results?
Yes, diet (e.g., high iodine intake), stress, or certain medications (like thyroid medications or blood pressure drugs) can influence T4 levels. Your doctor may recommend adjustments to your diet or lifestyle if needed.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

Medical Disclaimer

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