What are thyroid function tests?

Thyroid function tests are used to check for thyroid problems. These tests measure your blood levels of:

• thyroid stimulating hormone (TSH)
• thyroxine (T4)
• triiodothyronine (T3)

The pituitary gland in your brain makes TSH and sends it to your thyroid gland. TSH controls the production of thyroid hormones T3 and T4.

TSH is usually tested first. If your results are outside of the normal range (too high or too low), your doctor may request tests for T3 and T4. A test that checks your levels of TSH, T3 and T4 is known as a thyroid function test (TFT).

Additional tests your doctor may consider if they are concerned about a thyroid condition include:

• thyroid antibodies — to detect thyroid autoimmune conditions
• thyroid ultrasound — if nodules are found during physical examination
• thyroglobulin tests — to help detect thyroid cancer

When are thyroid function tests used?

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 low or too high levels of these hormones can make you unwell.

Sometimes your thyroid hormones can be outside of the normal range if you have a problem with another endocrine (hormone-producing) organ. Thyroid function tests can help find out if your problem is with the thyroid gland, rather than the pituitary or hypothalamus.

Find out more about thyroid conditions.

Your doctor might recommend thyroid function tests if you:

• have symptoms of hyperthyroidism (an overactive thyroid)
• have symptoms of hypothyroidism (an underactive thyroid)
• are taking thyroid hormone replacement treatment
• are female and experiencing infertility
• experience unexplained changes to your period
• have symptoms of other endocrine (hormonal) problems

All newborn babies in Australia are screened for some types of thyroid problems using the heel prick test. One in every 4,000 babies is born without a working thyroid gland.

Where can I get thyroid function tests?

You usually need to go to a pathology collection centre with a referral from your doctor. Your doctor will recommend one near you.

Sometimes, your doctor will collect blood for testing in their clinic. Blood tests are also routinely done in hospitals.

How do I prepare for thyroid function tests?

You don't need to prepare for a TSH blood test.

Tell your doctor if you are taking any medicines or supplements, especially medicine for thyroid problems.

If you are having other blood tests done at the same time, you may need to follow other instructions. Check with your doctor or the pathology collection centre if there is anything you need to do before your blood test.

What happens during thyroid function tests?

A health professional will take a sample of blood from a vein in your arm. The blood will then be sent to a laboratory for testing.

What do my thyroid function test results mean?

A high TSH result might mean that you:

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

A low TSH result might mean that you:

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

If your TSH test result isn't normal, your doctor may ask the lab to test your thyroxine (T4) and/or triiodothyronine (T3) levels.

Discuss your test results 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 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 to consider before having the test
• what happens during a blood test
• the accuracy of your test results
• the cost of your blood tests

You can call the healthdirect helpline on 1800 022 222 (known as NURSE-ON-CALL in Victoria). A registered nurse is available 24 hours a day, 7 days a week.