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Thyroid Hormone: Total Triiodothyronine (T3) in Serum or Plasma

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

This test measures the total amount of the hormone triiodothyronine (T3) in your blood. T3 is a key hormone produced by your thyroid gland that helps regulate metabolism, energy levels, and other bodily functions.

Quick summary

["Understanding your T3 levels can help assess thyroid function and overall health."]

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

Key facts at a glance

LOINC
12954-4
Short code
T3 sp3 p chal SerPl-mCnc
Synonyms
T3 sp3 p chal SerPl-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

What Is Triiodothyronine (T3)?

Triiodothyronine (T3) is a hormone produced by your thyroid gland. It plays a crucial role in controlling metabolism, growth, and development. While thyroxine (T4) is the primary hormone produced by the thyroid, T3 is more active in the body and helps convert T4 into its active form. Your body naturally converts some T4 into T3, and this process is essential for maintaining energy levels and proper bodily functions.

Why Might You Need This Test?

Your doctor may order a total T3 test if you have symptoms of thyroid dysfunction, such as fatigue, weight changes, or unexplained mood swings. It can also be used as part of a broader thyroid evaluation, especially when T4 levels are abnormal or when monitoring thyroid hormone levels over time.

How Is This Test Performed?

A blood sample is taken from a vein in your arm. The sample is then sent to a laboratory for analysis. The test measures the total amount of T3 in your serum or plasma, depending on the type of blood sample collected.

What Do the Results Mean?

Interpreting T3 results involves considering the context of your symptoms, other test results, and medical history. While specific reference ranges can vary by lab, lower than expected levels may suggest hypothyroidism (underactive thyroid), and higher levels may indicate hyperthyroidism (overactive thyroid) or other conditions. Your healthcare provider will discuss the results with you.

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 T3

Frequently asked questions

What causes abnormal T3 levels?
Abnormal T3 levels can result from various conditions, including thyroid disorders (such as hypothyroidism or hyperthyroidism), certain medications that affect thyroid hormone metabolism, or conditions that interfere with thyroid hormone production or conversion. Additionally, stress, illness, or pregnancy can temporarily alter T3 levels. Always consult your healthcare provider for a detailed explanation based on your specific situation.
How often should I get this test?
The frequency of T3 testing depends on your medical history, symptoms, and overall health. If you have a known thyroid condition, your doctor may recommend regular monitoring. If you are asymptomatic, testing may only be necessary if other thyroid tests indicate a need. Follow your healthcare provider’s guidance for the best schedule.
Can lifestyle changes affect T3 levels?
Yes, lifestyle factors such as diet, stress levels, sleep, and physical activity can influence thyroid hormone levels. Eating a balanced diet rich in iodine and selenium, managing stress, and maintaining healthy sleep habits may support optimal thyroid function. However, significant changes in T3 levels are more commonly linked to underlying medical conditions rather than lifestyle alone.
  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