T4 is converted to T3 in tissues
Biomarker library
Thyroid Health
Free T4
Free T4 (thyroxine) is the main hormone your thyroid releases. ‘Free’ means the small fraction that isn't bound to carrier proteins and can actually enter cells.
Measured in your health assessment, with a doctor to explain your result. Book yours
Why it matters
T4 is a reservoir hormone: your tissues convert it into the more active T3 as needed. Thyroid hormones set your metabolic pace, including heart rate, body temperature, energy use and even how quickly you think.
What a result can mean
Lower than the range
A low free T4 with a high TSH confirms an underactive thyroid. A low T4 with a normal or low TSH is unusual and can point to a pituitary problem.
Higher than the range
A raised free T4 with a low TSH confirms an overactive thyroid. It can also be raised by too much thyroid medicine, or for a while by inflammation of the thyroid (thyroiditis).
What can help
Thyroid conditions are treated according to their cause, and people taking levothyroxine have their dose guided by TSH and T4.
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 T4.
Measured in your health assessment and explained by the doctor who reviews your results.