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Triiodothyronine Total T3 Total

Also Known as:   Total Triiodothyronine, TT3, Total T3, T3 total

What is a T3 total test?

This T3, also known as triiodothyronine, is a thyroid hormone with multiple functions in the body. T3 regulates the body's metabolism, activates cells, and enables important organs to function.T3 has three iodine atoms attached to it. Thyroxine (T4), the other primary thyroid hormone, has four iodine atoms. The thyroid produces some of the T3 that circulates in your body. On the other hand, most of it starts as T4 and is changed to T3 in the blood by the removal of one iodine atom. As it circulates in the blood, more than 99% of T3 in the body is bound to proteins. Free T3 refers to the fraction of T3 that is not linked to proteins. The body regulates the interaction between bound and free T3 in an exact balance to ensure optimal physiological functions. The total T3 test measures both bound and free T3. 

What is the test used for? 

Thyroid function is evaluated by total T3 testing. Free and total T3 testing, along with other thyroid function tests, are used to diagnose thyroid disorders such as an overactive or underactive thyroid gland, as well as to assess the severity of thyroid problems, diagnose pituitary gland disorders, and monitor patients with known thyroid conditions. The total T3 test is often included in a thyroid panel, a group of tests used to assess thyroid health. If a patient has had an abnormal thyroid stimulating hormone (TSH) test, T3 testing might be used as a follow-up test.

Many studies suggest that total T3 is a more reliable measurement than free T3. Total T3 can be used to identify hyperthyroidism or a pituitary gland disease and determine the origin of an overactive thyroid and monitor a patient's response to hyperthyroidism treatment.

Why and when do you need this test?

When a doctor detects a thyroid problem, a total T3 test is often ordered. This could be due to an abnormal TSH test result or because the patient may be experiencing symptoms of hyperthyroidism (an overactive thyroid) that may include:

  • Weight loss 
  • Anxiety and irritability
  • Palpitations
  • Bulging eyes
  • Increased appetite
  • Mood swings
  • Irregular heartbeat
  • Goiter (enlarged thyroid gland)
  • Heat intolerance
  • Diarrhea
  • Hand tremors
  • Trouble sleeping
  • Irregular periods
  • Muscle weakness

What kind of sample is required for the test?

A healthcare provider will use a small needle to obtain a blood sample from a vein in your arm. A small amount of blood will be collected into a test tube or vial once the needle is inserted. When the needle goes in or out, it may sting a little. It usually takes less than five minutes to complete this process.

Do you need to prepare for the test?

A T3 blood test does not require any special preparation. If you need to stop taking any medications before your test, your doctor will let you know. T3 levels can be increased or decreased by taking certain medications. The following drugs may affect your T3 levels:

  • Thyroid related drugs 
  • Steroids 
  • Hormones-containing birth control pills or other drugs, such as androgens and estrogens
  • Certain herbal medicines

Are there any risks to this test?

There are no significant risks of having a blood test. You or your child may experience minor pain or bruising where the needle was inserted, but most symptoms disappear quickly.

What do the test results mean?

The results of your total T3 test will indicate whether your T3 is within the normal range, below normal, or higher. The reference ranges may differ slightly from one laboratory to another. The following reference ranges are often used:

Free T3: 2.3–4.2 picograms per milliliter (pg/mL)

Total T3: 80–200 nanograms per deciliter (ng/dL) 

If you have high total T3 or free T3 levels, you may have hyperthyroidism.  Low T3 levels could indicate hypothyroidism. Hypothyroidism is a disorder in which your body does not produce enough thyroid hormone. T3 test results are often compared to T4 and TSH test results to detect thyroid diseases. Conditions that impact the concentration of Thyroxine Binding Globulin (TBG) affect T3 levels as well. T3 levels can be slightly elevated during pregnancy or estrogen therapy. In contrast, they can be decreased with severe illness, malnutrition, renal failure, and during treatment with anti-thyroid medications, propranolol, propylthiouracil, and salicylates.
Related tests: Thyroid Panel, Free T4 Test, FreeT3 Test, Thyroid Antibodies, TSH 

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