Triiodothyronine, or T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4).  T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.

Many of your body’s cells can turn T4 into T3; T4 may be mainly a "reservoir" used to make T3 available.

About 99.7% of T3 in blood is attached to a specific protein, and the rest is unattached (free). Blood tests can measure either the total (both bound and unattached) or free T3 hormone in the blood. These tests are called total T3, and free T3 (fT3)  respectively. Most laboratories will only routinely measure and report FT3 results.