thyroid function test, any of several laboratory tests performed to evaluate the function of the thyroid gland. Thyroid function tests include protein-bound iodine, butanol-extractable iodine, T3, T4, free thyroxine index, thyroxin-binding globulin, thyroid-stimulating hormone, long-acting thyroid stimulator, radioactive iodine uptake, and radioactive iodine excretion. Often several of the tests are performed simultaneously.
