Thyroid function test – in-depth information

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, if you have a thyroid or pituitary condition 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, of the effects of thyroid hormones. T4 makes up nearly all of what is called thyroid hormone but is thought to have less effect. 

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

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
• are a woman with infertility problems
• are taking thyroid medicine, such as thyroxine, to monitor how you are responding to it.

Generally, you won't need to do anything before having this test. It can be done at any time of the day. Some medicines may affect the test so tell your doctor about all prescription and non-prescription medicines (such as herbal products, supplements and rongoā Māori) 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. 

A thyroid function test is a blood test which means a small amount of blood is taken through a needle placed in a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight for a few seconds. You may feel nothing at all from the needle, or you may feel a small brief sting or pinch. The blood sample is collected in a tube, which is sent to the laboratory for analysis. Read more about blood tests.

If a result in your thyroid function blood test is outside the normal range it can be flagged as high (H) or low (L). Interpretation of the many variations in test results is complex and an abnormal result may not mean that anything is wrong. Other health conditions, extreme stress and pregnancy affect the levels of thyroid hormones, as well as medicines. Talking with your doctor about what your results mean for you is important.

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.
• can also occur if you have an underactive thyroid gland and are receiving too little thyroid hormone medication
• can, in rare cases, indicate that you have a problem with your pituitary gland, eg, a tumour.

A low TSH result:

• can indicate an overactive thyroid gland or damage to your pituitary gland that is preventing it from producing TSH
• can also occur if you have an underactive thyroid gland and are receiving too much thyroid hormone medication.