T3 signals activity inside cells
Biomarker library
Thyroid Health
Free T3
Free T3 (triiodothyronine) is the most active thyroid hormone. Most of it is made in tissues such as the liver and kidneys by converting T4.
Measured in your health assessment, with a doctor to explain your result. Book yours
Why it matters
T3 is the hormone that binds to receptors inside your cells and turns up their metabolism. It helps diagnose an overactive thyroid, in which T3 can rise before T4.
What a result can mean
Lower than the range
A low free T3 is common during serious illness, fasting or strict calorie restriction, as the body conserves energy, and is seen in advanced underactive thyroid.
Higher than the range
A raised free T3 usually means an overactive thyroid. It can also rise when taking medicines that contain T3.
What can help
Free T3 is read with TSH and free T4, and treatment depends on the overall thyroid picture.
Read it with
NoteGeneral educational illustration. Colours, sizes and movement are simplified. This is not a scan or a simulation of your body. General information, not personal medical advice: a doctor reads your own results with your history.
Know your free T3.
Measured in your health assessment and explained by the doctor who reviews your results.