Free T3 (Triiodothyronine)

Your thyroid is a small, butterfly-shaped gland at the base of your throat. It makes two hormones, T3 and T4 (thyroxine). Thyroid hormones work together to control the way your body uses energy. They affect your weight, heart, body temperature, muscle strength, and even your mood.

There are two forms of T3 in your blood. Free T3 is the active form that enters your tissues where it's needed. Bound T3 is attached to certain proteins which prevent it from entering your tissues. A total T3 test measures both bound and free T3 together. A free T3 test only measures free T3.

Normal T3 levels alone do not rule out thyroid disease, so providers usually look at TSH and T4 along with T3.

High free T3 is often a sign of hyperthyroidism (an overactive thyroid). In some people, only the T3 level rises (a pattern called T3 toxicosis), while T4 stays normal — so checking T3 can help find this kind of hyperthyroidism that other tests might miss.

Low free T3 may be seen in hypothyroidism (an underactive thyroid). It can also drop in serious illness, starvation, or after surgery, even when the thyroid itself is fine — this is sometimes called 'euthyroid sick syndrome'.

Free T3 can also be helpful in pregnancy or in people with conditions that change the proteins that carry thyroid hormone in the blood, since the 'free' part of the test isn't affected by those proteins.