In brief
Blood tests that show how well your thyroid is working. TSH is usually measured first, and free T4 and free T3 are added if TSH is abnormal.
What it measures
Thyroid stimulating hormone (TSH) from the pituitary gland, and the free thyroid hormones T4 and T3.
Why it is done
To work out whether there is a problem with the thyroid, pituitary gland or hypothalamus, such as an underactive or overactive thyroid.
Reading the results
- High TSH with normal free T4 and T3: a mildly underactive thyroid, or the effect of an acute illness.
- High TSH with low free T4: an underactive thyroid.
- Low TSH with high or normal free T4 and T3: an overactive thyroid.
- Low TSH with low free T4 and T3: an underactive thyroid caused by a pituitary or hypothalamus problem.
TSH is the only thyroid test covered by Medicare if you do not have a history of thyroid problems.
Before the test
Do not take supplements containing biotin for two days before the blood test.
What can affect the result
- Thyroid hormone medicine
- Some prescription and over-the-counter medicines
- Biotin supplements
- Acute illness
Related topics
Connections
Thyroid function tests (TSH, free T4, free T3) measures
Testing TSH, free T4 and free T3 shows whether there is a thyroid or pituitary problem.
Sources: Pathology Tests Explained, Thyroid stimulating hormone (TSH); Pathology Tests Explained, Thyroid function tests
A high TSH can mean too little thyroid hormone is being produced.
Sources: Pathology Tests Explained, Thyroid stimulating hormone (TSH); Pathology Tests Explained, Thyroid function tests
Sources
Test information summarised from the sources listed on this page, read on 13 and 19 October 2026. General education only, not medical advice. Spotted a mistake? Please tell us through the HuBu-Intel contact page.