Free T4

Also written as: Free thyroxine, FT4

The unbound, active form of the main thyroid hormone.

What it measures

The portion of thyroxine circulating unbound to carrier proteins, which is the fraction available to act on tissues.

Why a doctor orders it

Usually ordered alongside or after TSH, to distinguish where in the system a problem sits and to confirm what an abnormal TSH suggests.

What is generally associated with a higher result

Broadly associated with an overactive thyroid, and with excess thyroid hormone replacement.

What is generally associated with a lower result

Broadly associated with an underactive thyroid, and with pituitary conditions.

Everyday things that move this number

Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:

Biotin supplements
Pregnancy
Some medications
Acute illness

Questions to take to your appointment

How does this read together with my TSH?

Build a printable questionnaire with your own results →

Where to read more

MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .

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