Free or total triiodothyronine (free T3 or total T3) test is used to assess thyroid function. It is ordered primarily to help diagnose hyperthyroidism and may be ordered to help monitor the treatment of a person with a known thyroid disorder.
FT3 can assist in determining whether the thyroid is performing properly, and is used mainly to help diagnose hyperthyroidism since T3 can become abnormal earlier than T4 and return to normal later than T4. This test may also be used for monitoring patients on T3 therapy.
Increased or decreased thyroid hormone results indicate that there is an imbalance between the body’s requirements and supply, but they do not tell the health practitioner specifically what is causing the excess or deficiency.
A high free T3 result may indicate an overactive thyroid gland.
Low free T3 results may indicate an underactive thyroid gland.