T4 Test | What is a t4 test (thyroid disease)?

 What is a T4 test?

A thyroxine (T4) lab test is used to check for thyroid problems, such as hyperthyroidism or hypothyroidism. T4 is the primary hormone produced by your thyroid, and plays a critical role in many of your body’s functions – such as hormone balance, metabolism, internal thermostat, and growth. It is usually done in conjunction with other thyroid tests, which are known a Thyroid Panel.

T4 levels are a primary indicator of how healthy your thyroid is. The amount of T4 production is controlled by your Pituitary gland. You pituitary gland tells sends messengers (called thyroid-stimulating-hormone TSH) to your thyroid when it wants the thyroid to produce more T4. If the (healthy) thyroid does not receive messengers, it slows down production of T4.

There are two types of T4 tests:

  1. A free T4 test (fT4)
  2. A total T4 test

Unfortunately, the “free” in free T4 does not refer to the cost of the test. When your body produces T4, the T4 needs help being transported around the body. Free T4 is just T4 that simply hasn’t found (bound to) a vehicle (the protein: thyroxine-binding globulin TBG)  that transports them around the body. High fT4 indicates that your body is producing a high amount of T4 because there is way more T4 than your body can handle – i.e., not enough TBG to carry around T4. 

When we talk about T4 test, we are talking about total T4: bound T4 and free T4. A total T4 test may also be referred to as just a “T4 Test” or “thyroxine test”.

WHAT IF T4 Levels Are low Or High?

If too little T4 is produced, then it means your thyroid is not producing enough thyroid hormone. This is known as hypothyroidism. See this article for a detailed overview of Hypothyroid symptoms, causes, and regaining health.

There are several primary reasons why T4 levels would be low:

  • Thyroid is unable to produce T4 because of inherent problems (such as from surgery, environmental toxins, or genetics)
  • Thyroid is unable to produce because of autoimmune disorder – your autoimmune system attacks thyroid’s functioning (such as with Hashimoto’s thyroiditis)
  • Pituitary gland is not telling your thyroid to produce adequate amount of T4

People with low T4 levels (hypothyroid) often have struggle with chronic lethargy (always tired), trouble stopping weight gain, hair loss, and feeling cold for no reason.

If too much T4 is released by your, it means your thyroid is overactive. This is known as hyperthyroidism. Hypothyroidism is the exact opposite of hypothyroidism, with the exact opposite symptoms as well. Hyperthyroidism is much less common than hypothyroidism.


PURPOSE OF A T4 TEST and how to prepare

The main purpose of a T4 test is, of course, to check the health of your thyroid! Usually, people with low/high thyroid tests will also experience high/low TSH levels. For more information about TSH, we discuss it in the above linked hypothyroid blog article, as well in this article about TSH and TSH tests.


  • If you have high TSH, you would do a T4 test to look out for low T4 levels as an indicator of hypothyroidism
  • If you have low TSH, you would do a T4 test to look out for high T4 levels as an indicator of hyperthyroidism

Of course, T4 tests are almost always (and should be) done in conjunction with other thyroid tests. These tests, which are done together, are know an Thyroid Panel Lab Tests. They will give you an accurate picture of what your thyroid is up to (or not up to!)

The best thing you can do to prepare for a test is be aware that there are medications and supplements that can skew thyroid testing results. Talk with your medical practitioner to see what – if any – medications you may need to stop taking before a thyroid panel in order to ensure accurate test results.



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Categories : Thyroid disease