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Thyroid Function Tests


What are thyroid function tests?

Thyroid function blood tests are performed to check the levels of hormones produced by your thyroid gland through your blood. There are some other thyroid functions tests, also which check the level of hormones produced by the pituitary gland in your brain that also act on the thyroid gland.

Why are these thyroid functions tests performed?

These tests are performed to evaluate the functioning of the thyroid gland and to diagnose the thyroid-related diseases and their causes. Sometimes our thyroid gland produces too many or too little thyroid hormones. Production of too many hormones in the bloodstream may cause hyperthyroidism which leads to increased metabolic rate, weight loss, sweating, rapid heart rate, and high blood pressure, among other symptoms. On the other hand, too little production of hormones can cause hypothyroidism that slows down bodily functions which leads to fatigue, weight gain, cold intolerance, and related symptoms.


Tests that are performed to assess thyroid function

Generally, TSH is the first blood test that is performed. TSH is the key hormone to diagnose hyperthyroidism and hypothyroidism. IF the results of TSH test are abnormal, there are a few more tests that need to be performed to determine the cause of the problem. Other tests, T3, T4, and free T4 are widely used tests.

There are some tests to evaluate thyroid functioning that include:

TSH Tests

It is common to start your thyroid function tests with a TSH test to measure the TSH level in a blood sample. High TSH level indicates that your thyroid gland is failing due to the problem affecting your thyroid directly, which is called primary hypothyroidism. While when TSH level is low, this shows the thyroid gland is overactive and producing too many hormones which is called hyperthyroidism. Sometimes, abnormality in the pituitary gland may lead to low TSH which prevents it from making enough TSH to stimulate the thyroid. This is called secondary hypothyroidism. A normal TSH indicates that your thyroid is functioning well. (But, what is normal for some, may not be normal for you. Listen to your body and your symptoms.)

T4 Tests (Thyroxine Test)

In blood, T4 is circulated in blood in two forms a) T4 bound – to proteins that prevent the T4 from entering the tissues that need thyroid hormone. b) Free T4 – which enters the several target tissues to exert its effects. T4 fraction is the most important to tell you how your thyroid is working and tests to measure this are called FT4 or Free T4 and Free T4 Index or FT4I. Those who have hyperthyroidism will have an elevated FT4 or FTI and those with hypothyroidism will have a low level of FT4 or FTI. By combining a TSH test with FT4, you can determine how your thyroid is working.

T3 Tests

This test is ordered for various reasons. The T3 test checks for levels of triiodothyronine, are usually ordered if T4 tests and TSH tests suggest hyperthyroidism. The T3 test may also be ordered if you are showing signs of an overactive thyroid gland. It can help diagnose the severity of hyperthyroidism. Patients suffering from hyperthyroid will have an elevated T3 level. In some patients with low TSH, only the T3 is elevated and the FT4 or FTI is normal. It is also possible to have severe hypothyroidism with a high TSH and low FT4 or FTI with normal T3. During pregnancy or while taking birth controlling pills, high levels of T4 and T3 can exist. (Always ask for thyroid blood work before, during and after pregnancy.)


Thyroid Antibody Tests

Thyroid Antibodies are present in Hashimoto’s disease which is the most common cause of hypothyroidism. Antithyroid antibodies work as a marker in the blood and their presence helps in diagnosing Hashimoto’s disease.

There are two principal types of antithyroid antibodies:

  • anti-TG antibodies that attack a protein in the thyroid which is called thyroglobulin.
  • anti-thyroperoxidase, or anti-TPO, antibodies, that attach enzyme in thyroid cells.

Thyroid-stimulating immunoglobulin (TSI) Test 

This test measures the amount of thyroid-stimulating immunoglobulin in your blood.

TSI is an autoantibody present in Graves’ disease which is the most common cause of hyperthyroidism. TSI imitates TSH by stimulating the thyroid cells which causes the thyroid gland to secrete excess hormone.

Radioactive iodine uptake (RAIU) 

It is a non-blood test. Because T4 contains so much iodine, the thyroid gland must take out the excess amount of iodine from the bloodstream, in order to enable the gland to make a sufficient amount of T4. The thyroid gland has developed a very active mechanism for doing so. Therefore, you can measure this activity by making an individual swallow a small amount of iodine, which is radioactive. This will help your doctor to know where the iodine molecules are going. By measuring the amount of radioactivity, doctors can know if the gland is functioning normally. High RAIU is seen in people with overactive thyroid gland. While low RAIU is seen in the people with underactive thyroid gland.


If your blood work suggests that your thyroid gland is overactive or underactive, your doctor may order a thyroid uptake test or an ultrasound test to ensure that the problem is not due to a structural problem with the thyroid gland or a tumor.

If the scan is normal, your doctor will probably prescribe medication to regulate your thyroid activity and will follow up with additional thyroid function tests to make sure the medicine is working.


**This article originally featured on Hashimoto-Disease.com**

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