What are thyroid function tests?

Thyroid function tests are blood tests that are used to evaluate the function of the thyroid gland. These blood tests may include measurements of the hormone that regulates the function of the thyroid gland, as well as direct measurements of the different forms of thyroid hormone produced and secreted by the thyroid gland. Thyroid function tests may include the following.

TSH - Also known as thyroid stimulating hormone or thyrotropin. This is the hormone secreted by the pituitary gland that regulates the function of the thyroid gland.

T4 - Also known as thyroxine. This is the major form of thyroid hormone produced by the thyroid gland.

T3 - Also known as triiodothyronine. This is the other form of thyroid hormone produced by the thyroid gland.

Free T4 - Also known as free or unbound thyroxine. This is the fraction of T4 that is not bound to protein in the bloodstream.

Free T3 - Also known as free or unbound triiodothyronine. This is the fraction of T3 that is not bound to protein in the bloodstream.

T3RU - Also known as the T3 resin uptake. This test provides an indirect estimate of the degree to which T4 is bound to protein in the bloodstream.

FTI - Also known as the free thyroxine index. This test is a calculation that provides an indirect estimate of the fraction of T4 that is not bound to protein in the bloodstream.