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Thyroid Hormone, Total Serum

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

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

Quick summary

["Your doctor may check T4 levels to assess thyroid function or diagnose conditions like hypothyroidism or hyperthyroidism."]

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

Key facts at a glance

LOINC
12925-4
Short code
T4 sp4 p chal SerPl-mCnc
Synonyms
T4 sp4 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 This Test Is Done

Your doctor might order a total T4 test if you have symptoms like fatigue, weight changes, or unexplained heat sensitivity/cold intolerance. It can also be part of routine thyroid function testing or monitoring for thyroid disorders.

What to Expect Before the Test

No special preparation is needed, but avoid taking thyroid medications 24 hours before the test if possible. Bring any medications to your appointment.

What the Test Involves

A healthcare provider will draw blood from a vein in your arm. You may feel a slight pinch, but it usually takes only a few minutes. The blood sample is sent to a lab for analysis.

What the Results Mean

Results help determine if your thyroid is working normally or if it’s overactive or underactive. Your doctor will compare your results to standard ranges and discuss them with you.

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.

Total T4 is a critical marker for thyroid function. Low levels may indicate hypothyroidism (underactive thyroid), while high levels can suggest hyperthyroidism (overactive thyroid). It’s often measured alongside thyroid-stimulating hormone (TSH) to get a full picture of thyroid health.

Short name

Total T4 (Thyroxine)

Frequently asked questions

What causes abnormal T4 levels?
Abnormal T4 levels can result from thyroid disorders (e.g., Hashimoto’s disease or Graves’ disease), thyroid nodules, pituitary issues, or medications that affect thyroid hormone production.
How often should I get this test?
Your doctor will determine the frequency based on your symptoms, medical history, and other tests. Regular monitoring may be needed if you have a thyroid condition or take thyroid medication.
Can diet affect T4 test results?
Yes, certain foods (like soy products) or supplements (e.g., selenium) may temporarily affect thyroid hormone levels. Avoid extreme dietary changes before testing, but discuss this with your doctor.
  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