About this test
T4 (thyroxine) is the main hormone made by the thyroid gland. It is changed into the more active T3 in the body. T4 is checked together with TSH and T3 to find out whether the thyroid is underactive or overactive.
Why it is done
- To help diagnose an underactive or overactive thyroid.
- To look further into an abnormal TSH result.
- To monitor thyroid treatment.
Preparation
Fasting is not usually required. Tell us about thyroid medicines or supplements such as biotin.
This information is general and is not medical advice. Please speak to your doctor about your results and treatment.