What are thyroid function tests?

Thyroid function tests are used to check for thyroid problems. The tests measure the amount of thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) in your blood.

The pituitary gland in your brain makes TSH and sends it to your thyroid gland. TSH controls the production of thyroid hormones.

TSH is usually tested first. The combination of a TSH test and tests for the thyroid hormones T3 and T4 is known as a thyroid function test (TFT).

Additional thyroid tests include thyroid antibodies.

Why would I need thyroid function tests?

Your thyroid gland has an important role in controlling your metabolism. Hormones made by your thyroid gland influence your:

• heart rate
• blood pressure
• body temperature
• weight

Too much or too little of these hormones can make you ill.

You might need thyroid function tests if you have symptoms of:

• hyperthyroidism (an overactive thyroid)
• hypothyroidism (an underactive thyroid)
• you are taking thyroid hormone replacement treatment
• you are female and being investigated for infertility

Signs of hyperthyroidism (an overactive thyroid) are:

• sensitivity to heat
• weight loss
• fast heartbeat
• nervousness
• sweating
• increased frequency of stools (pooing)

Signs of hypothyroidism (an underactive thyroid) are:

• tiredness
• weakness
• weight gain
• muscle aches
• constipation
• slow heart rate
• sensitivity to cold

All newborn babies in Australia are screened for thyroid problems. This is done by a drop of blood taken from a heel prick. One in every 4000 babies is born without a working thyroid gland.

How to prepare for thyroid function tests

There is no preparation needed for a TSH blood test.

Tell your doctor if you are taking any medicine, especially medicine for thyroid disorders.

Understanding your results

A high TSH result might mean that you:

• have hypothyroidism (an underactive thyroid gland)
• have a problem with your pituitary gland
• are receiving too little thyroid hormone medicine

A low TSH result might mean that you:

• have hyperthyroidism (an overactive thyroid gland)
• have a problem with the pituitary gland
• you are taking too much thyroid hormone medicine

If your TSH test result isn’t normal, your doctor will often ask for your thyroxine (T4) and/or triiodothyronine (T3) levels to be tested.

Discuss the results of your tests with your doctor to find out what they mean for you.

Resources and support

About thyroid function testing

See Pathology Tests Explained for more information about the TSH, T3 and T4 blood tests and thyroid problems.

About thyroid disorders

Read more about the thyroid and thyroid problems at the Australian Thyroid Foundation.

About blood testing

Visit healthdirect’s 'Guide to blood testing' to learn more about blood tests in general such as:

• what happens during a blood test
• the accuracy of your test results
• the cost of your blood tests