Skip to content

Thyroid hormone total (T4) concentration in 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, in your blood.

Quick summary

["Your thyroid hormone levels help regulate metabolism, energy, and overall bodily function."]

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

Key facts at a glance

LOINC
14014-5
Short code
T4 PA Bnd SerPl-mCnc
Synonyms
T4 PA Bnd 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

Doctors may order this test to check for thyroid disorders, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid). It can also help monitor treatment for thyroid conditions or evaluate symptoms like fatigue, weight changes, or mood swings.

What to Expect Before the Test

No special preparation is needed. You may have a blood draw from a vein in your arm. If you’re on thyroid medication, ask your provider whether to stop it temporarily before testing.

What the Results Mean

Results help compare your hormone levels to general reference ranges, but they’re interpreted with other thyroid tests (like TSH) and your symptoms. Abnormal results may signal thyroid dysfunction.

Common Conditions Linked to T4 Levels

Low T4 levels can indicate hypothyroidism, while high levels may suggest hyperthyroidism or thyroid dysfunction. Other conditions, like pregnancy or certain medications, can also affect results.

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

T4 Total

Frequently asked questions

What happens if my T4 test is abnormal?
If your T4 levels are outside the expected range, your doctor may recommend additional tests (like TSH or free T4) or further evaluation to determine the cause. They may also adjust medications if needed.
Can diet or supplements affect my T4 test?
Certain foods (like soy or goitrogens) or supplements (e.g., selenium) may influence thyroid function. Avoiding them for a few days before testing can help ensure accurate results.
How often should I get this test?
Your doctor will decide based on your health needs—some people get it yearly, while others may need it more often if they have ongoing thyroid issues.
  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