T3 Test

What is T3 (Triiodothyronine)?

T3 is a thyroid hormone. Full form of T3 is triiodothyronine .It is an important hormone of thyroid.

T3 is of two types- free T3 and bound T3. Bound T3 is the Protein bound T3. Free T3 is unbound to protein.

Total T3 is the sum of free T3 and bound T3.

T3 regulates the metabolism of our body and helps the body to perform the essential functions properly that are vital for a healthy body.

Why T3 test needed?

T3 test (free T3, total T3) are needed if there is any suspect that thyroid gland is not functioning properly, so to evaluate the thyroid function and to know if there is any thyroid disorder is present or not.

T3 test is also needed in those who are more prone or at higher risk to develop thyroid disease such as

  1. Type 1 diabetes
  2. Pernicious anemia
  3. Family history of thyroid disorder
  4. Primary adrenal insufficiency, or Addison’s disease
  5. Older adults

T3 Test Purpose

Purpose of T3 test are the following 

  1. To evaluate thyroid function
  2. To diagnose any thyroid disorder ( such as underactive or overactive thyroid)
  3. To know the severity of the thyroid condition
  4. To diagnose pituitary gland disorder
  5. To know the appropriate treatment of the patient 

T3 Test Importance

Importance of T3 test are the following

  1. To determine the right dose of thyroid medicine
  2. To predict the treatment outcome
  3. To monitor the patients with already diagnosed thyroid disorder
  4. To know the treatment response
  5. To determine if any change of treatment is needed or not
  6. To follow up the patient if TSH test result is abnormal

Free T3

T3 test can be either free T3 test or Total T3 test.

Free T3 test is used to determine that how effective is the thyroid replacement therapy. Also, it helps to diagnose if there is any abnormalities of proteins that bind the T3.

Total T3 Thyroid

Total T3 is more reliable than free T3 for diagnosis of thyroid problems. Total T3 helps to diagnose hyperthyroidism, pituitary gland disorder, determine or evaluate the cause of overactive thyroid, monitor treatment response of hyperthyroidism.

T3 and T4

T3 is also known as triiodothyronine. It is a thyroid hormone just like T4.  The major difference between T3 and T4 is T3 has three iodine atoms that are attached to it and T4 has four iodine atoms that are attached to it. 

Some T3 is made by thyroid gland, but majority of circulating T3 are formed from T4 by the removal of one iodine atom.

With T4, the T3 plays vital role for regulating metabolism of body, activation of cells and for proper functioning of vital organs. Both T4 and T3 helps to control body weight, muscle strength, body temperature, heart rate and many more functions.

More than 99 percent of T3 are protein bound and circulates in the blood stream in this bound form. Only the unbound T3 that are not bound to the protein (free T3) is able to act on cells, thus stimulate many vital physical processes. Without these essential physical processes, a person can not lead a healthy life. Free T3 and bound T3 are regulated by body in such a proper way that a fine balance is maintained between the two, so that proper body function is maintained.

T3 tests are done along with other thyroid functions test such as TSH, free T4, total T4 for the proper evaluation of thyroid function.

When Should I Get a T3 Test?

Usually a T3 test is needed if there is any suspect of thyroid disorder. T3 test is a part of routine thyroid test like TSH, T4.

T3 test is needed if the TSH test result is not within normal range or there are symptoms of hyperthyroidism.

T3 is less useful to diagnose hypothyroidism because T3 is typically the last hormone that becomes abnormal in hypothyroidism. So, it is not a very useful guide in patients with hypothyroidism.

T3 Test Procedure

Just like TSH, T4 blood test, T3 test is done by taking blood from vein (preferably vein of arm). The blood sample is then be analyzed and based on this blood sample T3 test result is prepared.

Drugs that Affect T3 Test Results

Following are the drugs that can affect the free T3 or total T3 test results

  1. Female hormone estrogen
  2. Birth control pills
  3. Steroids
  4. Methadone
  5. Male hormones ( androgens)
  6. Lithium
  7. Propranolol 
  8. Some herbs and supplements 
  9. Thyroid related drugs

It is essential to talk with your doctor if you are taking the above mentioned medicines before going for the test. You may need to discontinue for few days before the test. Do not discontinue the medicine without consulting with your doctor. Follow the doctor’s instructions.

T3 Levels

Normal range of T3- 

Free T3 normal range– 2.3 to 4.2 picograms per milliliter (pg/mL)

Total T3- 80 to 180 nanograms per deciliter ( ng/dL)

High total T3 or high free T3 simply means overactive thyroid or hyperthyroidism. Similarly, low T3 level means underactive thyroid or hypothyroidism.

T3 Test Result Influencing Factors

As per the experts, total T3 is more reliable than free T3, especially to assess underactive thyroid (hypothyroidism). It is thought because many factors can influence the free T3 level in blood such as

  1. Pregnancy
  2. Disease
  3. Drugs
  4. Different laboratory methods to measure free T3. So, it is found that same blood sample gave different test result based on different methods used.

T3 Level High

High T3 levels are usually common in pregnant women and in liver disease.

High T3 levels are also seen in

  1. Hyperthyroidism 
  2. Graves’ disease
  3. Painless thyroiditis 
  4. Toxic modular goiter
  5. Thyrotoxic periodic paralysis 
  6. Rarely, thyroid cancer or thyrotoxicosis.

Low T3

Low T3 level is seen in

  1. Hypothyroidism 
  2. Starvation 
  3. Long term illness
  4. Underactive thyroid due to autoimmune disease, radiation, or thyroid surgery

Do I Need Thyroid Follow Up Tests?

If total T3 or free T3 is higher than normal, additional tests may be needed to know the reasons for the overactive thyroid. The tests are

  1. Thyrotropin receptor antibody ( TRab) test- it is a blood test. It confirms or rules out the presence of Graves’ disease.
  2. Thyroid scan- it is an imaging test. It helps to detect enlarged or inflammed thyroid gland.
  3. Radioactive iodine uptake- it is an imaging test. It measures the rate at which the thyroid gland absorbs radioactive iodine in a specific time period.

Thyroid scan and radioactive iodine uptake tests reveal if the whole thyroid gland is affected or only a part of the gland is affected. This affected part of thyroid gland is also called as nodules.

T3 Test and Pregnancy

T3 test is usually not needed during pregnancy as it is normal to have thyroid hormone level change during pregnancy. Most of the cases, this is not serious issue. But T3 test may be needed in the following conditions 

  1. Presence of thyroid disorder symptoms 
  2. History of thyroid disease
  3. Presence of autoimmune disease
  4. Family history of thyroid disease

T3 Test Results


T3 test is a part of routine thyroid tests. Many factors can influence T3 test results as mentioned above. So proper interpretation of test result is very important before concluding a final diagnosis of thyroid disorder.

T3 test results must be correlated with other thyroid function tests like TSH, T4. Solely based on T3 test no final diagnosis about thyroid disorder should be given. Similarly, normal T3 test result does not mean that thyroid is functioning properly. T3 can be normal when T4, TSH level can be abnormal.

So, it is important to do all the thyroid function tests and based on the test results receive the proper treatment. Stay healthy and happy.


  1. Goldman-Cecil Medicine
  2. Williams Textbook of Endocrinology E-Book
  3. Guyton and Hall Textbook of Medical Physiology
  4. Davidson’s Principles and Practice of Medicine 23rd edition
  5. Harrison’s Principles of Internal Medicine 20th edition
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