Free T3
Also written as: Free triiodothyronine, FT3, T3 free
The free, active form of T3, the more potent of the two main thyroid hormones.
What it measures
The unbound (free) portion of triiodothyronine, or T3, the more active of the two main thyroid hormones. Most T3 in blood is attached to carrier proteins; this test measures only the small free fraction available to act on cells.
Why a doctor orders it
Usually ordered after TSH and free T4 when an overactive thyroid is suspected, because some overactive states raise T3 more than T4. It is generally less useful when an underactive thyroid is being investigated.
What is generally associated with a higher result
Broadly associated with an overactive thyroid gland, with thyroid hormone medicines, and with biotin supplement interference on some laboratory methods.
What is generally associated with a lower result
Broadly associated with an underactive thyroid, but also - very commonly - with recent significant illness, fasting, or strict dieting, because the body deliberately turns down T3 during illness. This pattern, often called non-thyroidal illness, is not a thyroid disease.
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 (vitamin B7) supplements, which can distort results on some lab methods
- Any recent significant illness, which lowers T3 as a normal response
- Fasting or strict dieting in the days before the draw
- Thyroid medicines and the timing of the last dose
- Pregnancy
Questions to take to your appointment
- How does this fit with the TSH and free T4 from the same draw?
- I take biotin - should it be paused before a repeat test?
- Could a recent illness or dieting explain a lower T3?
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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