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Thyroid hormone total concentration (serum/plasma)

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.

Quick summary

["Total T4 helps assess thyroid function and identify potential hormone imbalances."]

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

Key facts at a glance

LOINC
31145-6
Short code
T4 DBS-sCnc
Synonyms
T4 DBS-sCnc
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 this test to check for thyroid disorders, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also be used to monitor treatment for thyroid conditions or evaluate symptoms like fatigue, weight changes, or mood swings that may relate to thyroid issues.

What to Expect Before the Test

No special preparation is usually needed, but you may be asked to fast for 8–12 hours before the test if your blood is drawn early in the morning. Wear a comfortable shirt for easy access to your arm.

How the Test Is Performed

A healthcare professional will insert a needle into a vein in your arm to draw a small amount of blood. The blood sample is then sent to a lab for analysis.

What the Results Mean

Results are compared to standard reference ranges, but your actual values will be interpreted by your provider based on your medical history and symptoms. Low or high levels may signal thyroid dysfunction that requires further evaluation.

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

What causes abnormal total T4 levels?
Total T4 levels can be affected by thyroid gland disorders (e.g., Hashimoto’s disease, Graves’ disease), thyroid surgery or radiation treatment, pregnancy, certain medications (like amiodarone or lithium), or thyroid-stimulating hormone (TSH) imbalances. Illnesses or stress may also temporarily influence results.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your healthcare provider will discuss the findings with you during your follow-up visit.
Can diet or supplements affect total T4 test results?
Some medications (e.g., soy products, selenium supplements, or certain thyroid medications) may interact with thyroid hormone testing. Avoid high-soy foods or supplements 2–3 days before testing if your provider recommends it.
  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