Free T3 (tri-iodothyronine)

Ranges vary between laboratories (for example 3.1 to 6.8 or 3.5 to 6.5 pmol/L) and are a little lower in older age and in pregnancy.

T3 is the active form of thyroid hormone. Most of it is made in the body's tissues by converting T4, and only a small unbound fraction circulates free. The free T3 test measures that fraction, which is what acts on the heart, muscles, gut and brain.

It is mostly requested when an overactive thyroid is being looked into, because T3 can be high before T4 is. Doctors also use it in people taking liothyronine, and some private test panels include it as standard.

• An overactive thyroid, including Graves disease and thyroid nodules
• Taking liothyronine or too much thyroid hormone replacement
• Some medicines, including amiodarone
• Higher in younger people in the general population

• An underactive thyroid, usually after T4 has already fallen
• Any acute or long-term illness, because the body converts less T4 to T3 when unwell
• Fasting and low-calorie diets
• Some medicines, including beta-blockers, steroids and amiodarone
• Older age, in the general population

• It falls during any illness, crash dieting or fasting and recovers afterwards
• Biotin supplements can interfere with the assay on some analysers
• Laboratories use different assays with different ranges

No fasting needed, but prolonged fasting or dieting lowers it. In someone taking liothyronine the value depends heavily on the time since the last tablet.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.