Thyroid function test

Thyroid function tests are blood tests to check if your thyroid gland is working properly. It is done to screen for and diagnose thyroid disorders, and to monitor treatment for hypothyroidism (underactive thyroid gland) or hyperthyroidism (overactive thyroid gland).

Key points:

  1. Thyroid function tests are blood tests to check if your thyroid gland is working properly. It is done to screen for and diagnose thyroid disorders, and to monitor treatment for hypothyroidism (underactive thyroid gland) or hyperthyroidism (overactive thyroid gland).
  2. Thyroid hormones help regulate the body’s metabolism (that is, how the body functions).
  3. There are several different types of thyroid function tests which may be carried out. Interpreting all the different tests is complicated as there are various conditions which can change the level of these hormones.
  4. Usually the first test to assess thyroid function is a blood test which measures the hormone called thyroid-stimulating hormone (TSH), in your blood.
  5. In addition to measuring TSH, thyroid function tests also check the levels of the thyroid hormones, triiodothyronine (T3) and thyroxine (T4), in the blood.
  6. To perform a thyroid function test, a blood sample is drawn from a vein in the arm, and collected in a tube. This tube is sent to the laboratory for analysis. 

What is a thyroid function blood test?

  • There are several different types of thyroid function tests which may be carried out.
  • The commonly ordered thyroid function blood test measures the level of thyroid stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4).
  • TSH (or thyroid stimulating hormone) is made by the pituitary gland located in your brain. TSH is the pituitary gland’s messenger — it tells the thyroid gland to start making thyroid hormone (T3 and T4). If too much T3 and T4 are being produced by the thyroid then this causes negative feedback on the pituitary gland and less TSH is produced. If too little T3 and T4 are being produced then TSH production increases. In healthy people this system regulates itself perfectly. However in thyroid or pituitary diseases the system gets unbalanced. 
  • T3 (or  triiodothyronine) is produced by the thyroid gland. It makes up less than 10% of what we call thyroid hormone, but it is potent and is thought to cause most, if not all, the effects of thyroid hormones.
  • T4 (or free thyroxine) is also is produced by the thyroid gland. T4 makes up nearly all of what we call thyroid hormone.

When is a thyroid function test done?

  • A thyroid function test is requested in a variety of situations, for example:
    • If you are suspected of having problems with your thyroid gland such as if you have signs or symptoms of an underactive thyroid gland (hypothyroidism) or an overactive thyroid gland (hyperthyroidism).
    • If you are suspected of having problems with your pituitary gland.
    • If you are taking medicines that can affect your thyroid function such as amiodarone and lithium.
    • If you are pregnant.
    • If you are suspected of having infertility problems (women).
    • To monitor your response if you are taking thyroid medication such as thyroxine.  

How to prepare for the test

For the most part, you do not need to do anything before having this test.

  • Some medicines may change the result of the thyroid function test. Tell your doctor about all prescription and non-prescription medication (such as herbal products) you take. 
  • If you are taking thyroid medicines, tell your doctor when you took your last dose. Your doctor may request that you to stop taking thyroid medicines temporarily before having this test.

How is the sample collected for testing

A blood sample is taken by a needle placed in a vein in the arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.The blood sample is collected in a tube, which is sent to the laboratory for analysis.

Results

Your doctor will discuss the results of your thyroid function test with you.
An abnormal TSH usually indicates a deficiency or an excess of thyroid hormones available to the body but it does not indicate the reason why. An abnormal TSH result is usually followed by additional testing of FT4 and/or FT3 to investigate the cause.

  • A high TSH result:
    • Often means an underactive thyroid gland (hypothyroidism).
    • In rare situations, a high TSH result can indicate a problem with the pituitary gland, such as a tumour.
    • Can also occur in people with underactive thyroid glands who have been receiving too little thyroid hormone medication.
  • A low TSH result:
    • Can indicate an overactive thyroid gland (hyperthyroidism) or damage to the pituitary gland that prevents it from producing TSH.
    • Can also occur in people with an underactive thyroid gland who are receiving too much thyroid hormone medication.

Learn more

The following is further reading that gives you more information on thyroid function tests. Be aware that websites from other countries may contain information that differs from New Zealand recommendations.

Blood test safety information Labtests NZ
Thyroid function tests Patient Info, UK
Lab tests Online  Australasian Association of Clinical Biochemists 

Credits: Editorial team.