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Thyroid hormone total serum concentration (T4)

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

This test measures the amount of the primary hormone produced by your thyroid gland, which helps regulate metabolism, energy levels, and growth.

Quick summary

["Your total T4 level helps doctors assess thyroid function and check for conditions like hypothyroidism or hyperthyroidism."]

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

Key facts at a glance

LOINC
35227-8
Short code
T4BG SerPl-msCnc
Synonyms
T4BG SerPl-msCnc
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 Is Done

Your doctor may order a total T4 test if you have symptoms like fatigue, weight changes, or unexplained hair loss. It’s also used to monitor thyroid treatment or screen for thyroid disorders before other tests.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the draw if your blood is collected early in the morning for consistency.

What Happens During the Test

A healthcare provider will draw blood from a vein in your arm. You may feel a slight pinch, but it usually doesn’t cause long-term discomfort.

What the Results Mean

Results alone don’t diagnose a condition—they’re used alongside other tests and symptoms. Your doctor will compare your level to what’s typical for your age and health.

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 T4

Frequently asked questions

When might my total T4 test be repeated?
If your results are outside the expected range or if you’re being monitored for thyroid disease, your doctor may schedule follow-up tests to track changes over time.
Could my medications affect this test?
Yes, certain medications (like some blood pressure or thyroid medications) can influence T4 levels. Your doctor will consider these when reviewing your results.
Is fasting required for this test?
No fasting is required, but timing (e.g., morning vs. afternoon) may affect results. Always follow your doctor’s instructions.
  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