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Thyroid function test – basic information

Key points about thyroid function tests

A thyroid function test is a group of tests that give information about how your thyroid is working.
It could be done to test for an underactive or overactive thyroid, or to monitor thyroid function if you're taking medicines.
It could also be done to check for a pituitary or fertility problem and as a screen in pregnant people and new-born babies.
This page is a short summary about thyroid function tests. For more detailed information, see thyroid function test – in-depth.

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A thyroid function test is a group of blood tests that give information about how your thyroid is working. The test measures the levels of:

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

Each substance gives different information. TSH and T4 tests are more commonly ordered, T3 is often only ordered if TSH and FT4 tests suggest you might have hyperthyroidism.

Thyroid function tests are usually performed to:

Help make a diagnosis if you have symptoms that could mean you have a thyroid disorder such as an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism) –
thyroid function tests are not usually required if you do not have symptoms.
Monitor your thyroid over time if you are taking medication.
Check for a pituitary problem.
Check for fertility problems.
Screen (check) for any thyroid problems in pregnant women and new-born babies.

Interpreting thyroid function test results is complicated and is best done in consultation with your healthcare team. They will know what is normal for you and how these results relate to your other signs and symptoms. An abnormal result may not mean you have anything to worry about.

Thyroid conditions and tests
Thyroid function test – in-depth information

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(external link) Labtests NZ
Thyroid function tests(external link) Patient Info, UK
Thyroid stimulating hormone (TSH)(external link) Australasian Association of Clinical Biochemists