Free triiodothyronine is the unbound form of the most biologically active thyroid hormone, most of it produced by conversion from thyroxine in the liver and other tissues rather than by the gland itself. It reflects how much active hormone reaches the cells. It is a supporting measure rather than a first-line thyroid test.
No readings for this marker yet.
Raised free T3 most often accompanies an overactive thyroid and can be the earliest abnormality in Graves' disease or a toxic nodule, occasionally rising while free thyroxine is still unremarkable. Lowered values are seen in an underactive thyroid, but far more often in serious illness, starvation or major surgery, where conversion to the active hormone slows without thyroid disease. Because of this it is rarely interpreted on its own. Repeat testing and a doctor's assessment settle what an isolated result means.
The sample is taken before the day's dose of thyroid hormone, and biotin supplements are usually stopped beforehand because they distort many immunoassays. Acute illness, fasting and marked weight loss depress the result independently of the gland, and it is unstable in blood left standing. It is interpreted alongside TSH and free thyroxine, with thyroid antibodies added when an autoimmune cause is being considered.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.