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Total Serum Thyroxine (T₄) by Mass Concentration

Published: August 9, 2026Updated: August 17, 2026

This test measures the amount of thyroxine (T₄), a hormone produced by your thyroid gland, in your blood serum.

Quick summary

["Your thyroid hormone levels help regulate metabolism and energy."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
26879-7
Short code
T3 Sal-mCnc
Synonyms
T3 Sal-mCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
80
Ref high
200
Critical range
20 N/A – 400 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
84436
SNOMED
302346007
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?

Doctors may order this test to check for thyroid disorders, such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), or to monitor thyroid function during treatment.

What Does a Result Mean?

Results are not provided with specific reference ranges. Your doctor will interpret the results in relation to your symptoms and other test results.

Common Reasons for Testing

It may be used if you have symptoms like fatigue, weight changes, or unexplained goiter (thyroid enlargement), or if you have conditions like Graves’ disease, Hashimoto’s thyroiditis, or pituitary disorders.

What to Expect Before the Test

No special preparation is usually needed, but your doctor may advise avoiding certain medications or dietary restrictions. You may be asked to fast for 8–12 hours if your blood is drawn early in the morning.

Reference range

Adult: 80-200 ng/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 T₄ results?
Abnormal results can occur due to thyroid diseases (e.g., thyroid nodules, thyroiditis), medications (e.g., amiodarone, lithium), pregnancy, or conditions affecting hormone metabolism. Your doctor will review your medical history and other test results to determine the cause.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on your lab and insurance. Follow-up with your healthcare provider to discuss findings and next steps.
Can diet affect T₄ test results?
Dietary changes alone rarely affect T₄ levels, but certain foods (e.g., soy, goitrogens) or supplements (e.g., selenium, iodine) may interact with thyroid function. Your doctor may recommend avoiding iodine-rich foods (e.g., seaweed) for 24 hours before testing if iodine levels are also being checked.
  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 17, 2026. · Read our editorial policy