Triiodothyronine (T3) is the more active of the two thyroid hormones produced by the thyroid gland. Most T3 is bound to protein in the blood. Free T3 measures the level of T3 that is free, or unbound to protein, and is available to regulate metabolism.
What might a low result mean?
Low levels of FT3 indicate that your thyroid is not producing enough thyroid hormones which means that you may have a an underactive thyroid. Symptoms of an underactive thyroid include gaining weight, feeling sensitive to cold, feeling tired, often to the point of exhaustion, and having dry skin and hair. In some people, their T3 is low because their body has difficulty in converting T4 to T3.
What might a high result mean?
Raised levels of free T3 can indicate an overactive thyroid. This happens when your thyroid gland produces too much thyroid hormone. Symptoms of an overactive thyroid include being sensitive to heat, losing weight, having a rapid heartbeat and feeling anxious and jittery. Some people may have high levels of free T3 if they are taking too much thyroid replacement hormone, especially if they are taking T3 (liothyronine) or a combination of T4 and T3 (e.g NDT).