What is T3 and T4

The thyroid gland produce two major hormones called Triiodothyronine (T3) and Thyroxine (T4). The thyroid gland takes iodine, found in our diet, combines it with an amino acid, tyrosine and converts it into T3 and T4.

  • T3 – Triiodothyronine: Containing three iodine atoms.
  • T4 – Thyroxine: Containing four iodine atoms
T3 Triiodothyronine Image
T4 Thyroxine Image

The Follicular cells are the major cell type in the thyroid gland and are responsible for the production and secretion of the thyroid hormones T3 and T4.

T3 (triiodothyronine): Approximately 20% of T3 is secreted into the bloodstream directly by the thyroid gland. The remaining 80% is produced by the removal of one iodine atom from T4 by a process called deiodination. This process occurs primarily in organs such as the liver and kidneys. T3 is the active form of the thyroid hormone T4 (thyroxine).

T4 (thyroxine): functions as a “storage” hormone. On its own, it is unable to produce energy. Therefore, it must undergo a process known as deiodination in which it loses an atom of iodine to become triiodothyronine (T3).

Both triiodothyronine (T3) and thyroxine (T4) are essential hormones for brain and physical development in infants and for metabolic activity in adults. Thyroid hormones help the brain, heart, liver, muscles, and other organs function properly.

How is T3 and T4 controlled?

The production and release of T3 and T4 is controlled by thyroid stimulating hormone (TSH) secreted by the pituitary gland, which is in turn regulated by thyrotropin-releasing hormone (TRH) secreted by the hypothalamus (a small region of the brain).

Negative Feedback Loop Image
Negative Feedback Loop

This process works as a negative feedback loop. When levels of T3 and T4 decrease below normal, the pituitary gland produces TSH, stimulating the thyroid gland to produce more hormones (T3 and T4). Once the levels rise, the pituitary gland then decreases TSH production.

Hence, this system allows the body to maintain a constant level of T3 and T4 in the body. 

Forms of T3 and T4 hormones.

Most of the T3 and T4 in your body are bound to serum protein such as ( thyroxine-binding globulin (TBG), transthyretin (TTR), albumin, and lipoproteins) and transported throughout the body.

However, a smaller portion of T3 and T4 doesn’t bind to protein is called Free T3 (FT3) & Free T4 (FT4), which circulates through the bloodstream as free or unbound hormones.

T3 exists in two forms in the blood:

  • Bound T3: More than 99.5% of T3 are bound to serum proteins that help transport the hormone through the body.
  • Free or Unbound T3: About 0.5% of T3 doesn’t bind to serum protein is called free T3 and circulates unbound in your blood.

T4 exists in two forms in the blood:

  • Bound T4: More than 99.95% of T4 are bound to serum proteins.
  • Free or Unbound T4: A minimal amount, about 0.02%, of T4 doesn’t bind to serum protein is called free T4 and circulates unbound in your blood.

Why doctors perform T3 and T4 tests

Blood tests for thyroid function such as T3, T4, TSH and others are an important part of diagnosing and treating thyroid disorders. Some conclusions can be drawn from a single test, however a combination of two or more tests are usually needed to check the health of your thyroid.

By comparing the values of different thyroid tests, a doctor can determine whether a person has hypothyroidism  (low thyroid function), hyperthyroidism (overactive thyroid), or an autoimmune thyroid disease such as Graves’ disease or Hashimoto’s thyroiditis.

T3 (triiodothyronine) Test

T3 (triiodothyronine) is a thyroid hormone and the test measures how much is in your blood.

  • Total T3 test: This test simply measures how much of T3 hormone both (bounded and free) is present in your blood. If the level is either abnormally high or abnormally low, it can be an indicator of thyroid disease.
  • Free T3 test: This test just measures free or unbounded T3 present in your blood. This test is often not reliable and therefore not typically helpful.
  • Reverse T3 test: Reverse T3 (RT3) is a biologically inactive protein that is structurally very similar to T3, but the iodine atoms are placed in different locations, which makes it inactive. RT3 is a blood test that not commonly ordered by the doctors.
Reverse T3 Image

What does T3 test results mean?

This single test may not give your doctor any definitive answers about what is wrong. However, abnormal results can help point them in the right direction. If your results show high T3 levels, it may mean you have hyperthyroidism (overactive thyroid). Low T3 levels may mean you have hypothyroidism, a condition in which your body doesn’t make enough thyroid hormone.

However, T3 tests are generally more useful for diagnosing hyperthyroidism than hypothyroidism.

Further, T3 tests are frequently ordered with T4 and TSH (thyroid stimulating hormone) tests, because it can give your doctor a more complete picture of what’s going on.

If you have questions about your results, talk to your health care provider.

T4 (thyroxine) Test

T4 (thyroxine) is a thyroid hormone and the test measures how much is in your blood.

The T4 test measures two key values:

  • Total T4 test: This test measures how much of T4 hormone (both bounded and free) is present in your blood. 
  • Free T4 test: This test just measures free or unbounded T4 present in your blood. A free T4 test is often preferred over a total T4 test.

What does T4 test results mean?

Your results may come in the form of total T4, free T4. High levels of any of these tests (total T4, free T4) may indicate an overactive thyroid, also known as hyperthyroidism. Low levels of any of these tests (total T4, free T4) may indicate an underactive thyroid, also known as hypothyroidism.

Therefore, if your T4 test results are not normal, your health care provider will likely order more thyroid tests to help make a diagnosis. These may include:

  • T3 thyroid hormone tests.
  • A TSH (thyroid stimulating hormone) test.
  • Tests to diagnose Graves’ disease, an autoimmune disease that causes hyperthyroidism.
  • Tests to diagnose Hashimoto’s thyroiditis, an autoimmune disease that causes hypothyroidism.

How do I get ready for T3 and T4 test?

It’s important to tell your doctor about all of the medications you’re currently taking, as some may affect your T3, T4 test results. If your doctor knows about your medications in advance, they can advise you to temporarily stop using them or consider their effect when interpreting your results.
Some medications that can affect your T3, T4 levels include:

  • Thyroid related drugs
  • Steroids
  • Birth control pills or other medications containing hormones, such as androgens and estrogens etc.

In addition, you may also need to fast (not eat or drink) for several hours before the test. Your health care provider will let you know if there are any special instructions to follow.

Procedure for T3, T4 test

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial (a small glass or plastic bottle).

Blood sample from a vein Image
Blood sample from a vein

You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Note: A normal T3 test result doesn’t necessarily mean that your thyroid is functioning perfectly. Measuring your T4 and TSH can help your doctor figure out if you have a thyroid problem despite a normal T3 result.

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