Thyroid function test

Thyroid function tests are blood tests to check whether your thyroid gland is working properly.

Key points

  1. Your thyroid is a small, butterfly-shaped gland in your neck. It produces thyroid hormones that help your body use energy, stay warm and keep your brain, heart, muscles and other organs working as they should.
  2. Thyroid function tests are done to screen for and diagnose thyroid disorders and to monitor treatment for hypothyroidism (underactive thyroid gland) and hyperthyroidism (overactive thyroid gland).
  3. There are several different thyroid function tests. Interpreting the tests is complicated as there are various conditions that can change the level of thyroid hormones.
  4. To perform a thyroid function test, a blood sample is drawn from a vein in your arm and collected in a tube. This tube is sent to the laboratory for analysis. 

What is a thyroid function blood test?

The most common thyroid function blood test measures the levels of thyroid-stimulating hormone (TSH), triiodothyronine (T3) and thyroxine (T4).

Thyroid stimulating hormone (TSH)

TSH is made by your pituitary gland, which is located in your brain. The pituitary senses the level of thyroid hormones in your bloodstream, just as the thermostat in your living room senses the temperature.

TSH is your pituitary gland’s messenger. If the level of thyroid hormones T3 and T4 drop just a little below normal, the pituitary reacts by secreting TSH, which tells your thyroid gland to produce more or less T3 and T4.

In healthy people, this system regulates itself perfectly. However, in thyroid or pituitary conditions the system gets unbalanced. 

Triiodothyronine (T3) and thyroxine (T4)

T3 and T4 are produced by your thyroid gland. T3 makes up less than 10% of what is called thyroid hormone, but it is potent and is thought to cause most, if not all, the effects of thyroid hormones. T4 makes up nearly all of what is called thyroid hormone but is thought have less effect. 

Usually, the 'free' or 'active' portion of T3 and T4 are measured, so these tests are known as FT3 and FT4.

When is a thyroid function test done?

A thyroid function test is requested in a variety of situations, including if you:

  • have signs or symptoms of an underactive thyroid gland (hypothyroidism) or an overactive thyroid gland (hyperthyroidism)
  • have signs of problems with your pituitary gland
  • are taking medicines that can affect your thyroid function, such as amiodarone and lithium
  • are pregnant
  • have infertility problems (women).

It is also used to monitor your response if you are taking thyroid medication, such as thyroxine

How should I prepare for a thyroid function test?

For the most part, you do not need to do anything before having this test. However, 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 ask that you 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 your arm. A tourniquet (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.

What do the results from my thyroid function test mean?

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 your body, but it does not indicate why this is happening.

An abnormal TSH result is usually followed by additional testing of FT3 and/or FT4 to investigate the cause.

A high TSH result:

  • often means an underactive thyroid gland (hypothyroidism)
  • can also occur if you have an underactive thyroid gland and are receiving too little thyroid hormone medication
  • can indicate in rare cases a problem with your pituitary gland, such as a tumour.

A low TSH result:

  • can indicate an overactive thyroid gland (hyperthyroidism) or damage to your pituitary gland that prevents it from producing TSH
  • can also occur if you have an underactive thyroid gland and 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: Health Navigator Editorial Team . Reviewed By: Gwenda Lawrence, medical laboratory scientist, Auckland Last reviewed: 11 Dec 2019