Free T3 blood test: normal range explained

T3, triiodothyronine, is one of the two main thyroid hormones. Most of it travels bound to proteins, which keeps it from entering tissues; the free T3 is the unbound part that can act on cells. A free T3 test measures only that free part, while a total T3 test measures bound and free together.

T3 is less than 10% of the thyroid hormone in the blood but about four times more powerful than T4, so it drives much of the hormone's effect on metabolism.

Doctors order T3 mainly to help diagnose an overactive thyroid and to find its cause, usually alongside TSH and T4, and to monitor treatment. It is not commonly used to check for an underactive thyroid, because TSH and free T4 pick that up earlier.

Free T3 is often requested after an abnormal TSH or free T4, when T3 toxicosis is suspected, or for someone on thyroid medicine who still has symptoms despite a normal TSH. MedlinePlus notes that experts consider total T3 the more accurate measure, and Testing.com says free T3 on its own does not identify thyroid disease.

Free T3 is sensitive to illness, pregnancy, biotin and several medicines, so the context around the draw matters.

• Being seriously ill, poorly nourished or recovering from surgery lowers free T3 without a thyroid problem, a pattern called non-thyroidal illness or sick euthyroid syndrome.
• Biotin supplements can give a falsely high or low result on the method labs use; Testing.com advises stopping biotin at least 72 hours before the draw.
• Amiodarone, corticosteroids, propranolol, lithium and anti-thyroid drugs can lower T3, some contrast dyes can affect it, and birth control pills, oestrogens and methadone can raise it. Do not stop a medicine unless your doctor tells you to.
• Pregnancy raises the proteins that carry T3, which changes total T3 more than free T3, and free T3 falls in the second and third trimesters, so trimester ranges apply.
• Fasting is not needed; eat and drink normally unless told otherwise.