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What is a Triiodothyronine (T3) test?
A T3 test is done to measure the amount of T3 hormone in your blood. T3 is produced along with T4 or thyroxin hormone by the thyroid gland in response to thyroid-stimulating hormone (TSH), a pituitary hormone. Some amount of T3 is also produced from T4 in body tissues.

Both these hormones help maintain various body functions including breathing, functioning of the heart and nervous system, controlling body temperature and cholesterol level and maintaining regular menstruation in women. They are available in two forms in the body:

  • Bound 
  • Free 

Overproduction of T3 or T4 will result in the onset of symptoms of hyperthyroidism, and their underproduction results in hypothyroidism. So, by assessing T3 levels, this test helps diagnose thyroid dysfunctions. 

  1. Why is a T3 test performed?
  2. How do you prepare for a T3 test?
  3. How is a T3 test performed?
  4. What do T3 test results indicate?

T3 test is done to assess thyroid function. It is recommended to individuals who show apparent symptoms of hypo or hyperthyroidism.

Some symptoms of hyperthyroidism are:

Symptoms of hypothyroidism include:

(Read more: Thyroid function test)

No special preparation is usually needed for this test. It can be done at any time of the day, and no fasting is required. However, some medicines can alter the results of a T3 test; hence, you must inform your doctor about the medications you are taking. He/she may advise you to stop taking certain medicines, such as birth control pills, clofibrate, oestrogen, methadone, certain herbal remedies, amiodarone, anabolic steroids, androgens, anti-thyroid drugs, lithium, phenytoin and propranolol. Also, the test should be avoided in case you are suffering from an acute illness.

To perform this test, a sample of blood will be drawn from a vein of your arm using a needle. The needle injection site will first be cleaned with alcohol before collecting the required amount of blood.

After the test, the injection site will be pressed firmly, and a cotton or gauze pad will be placed over that area to stop bleeding. The collected blood sample will be sent to a lab. for examination.

The whole process is painless and noninvasive, but few people experience slight redness in the area of needle insertion. The redness can be minimised by placing an ice pack over the affected area.

The test results can vary depending upon the method used and the age, gender, and health history of the person. A healthcare provider will provide the correct interpretation of your results.

Normal results: The normal range of free T3 hormone is 4-7.4 picomoles per litre (pmol/L)

Abnormal results: If the test result shows a higher than normal T3 level, it can be due to any of the following conditions:

  • Overactive thyroid gland 
  • T3 thyrotoxicosis (elevated serum T3 and normal T4 concentration)
  • Toxic nodular goitre (autonomously functioning thyroid nodules in a thyroid gland)
  • Liver disease
  • Intake of thyroid medicines or certain supplements 
  • First trimester of pregnancy
  • Intake of birth control pills or oestrogen

If the test result shows a lower than normal T3 level, it can be indicative of the following conditions:

  • Severe illnesses (short term or long term)
  • Thyroiditis (swelling or inflammation of the thyroid gland)
  • Starvation or malnutrition
  • Underactive thyroid gland

(Read more: T4 test results)

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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  1. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; Free and Bound Triiodothyronine (Blood)
  2. Alexander EK et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum Thyroid. 2017 Mar;27(3):315-389 PMID: 28056690
  3. Ross DS et al. 2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis Thyroid. 2016 Oct;26(10):1343-1421 PMID: 27521067
  4. HealthLink BC [Internet] British Columbia; Thyroid Hormone Tests
  5. Pennstate Hershey. [Internet] Milton S Hershey Medical Center, U.S. T3 test