Total thyroxine, free thyroxine and triiodothyronine are the types of thyroid blood tests. A total thyroxine blood test measures bound and free thyroxine. Free thyroxine tells how much total thyroxine is present, and triiodothyronine measure bound and free triiodothyronine, claims WebMD.
Free thyroxine affects bodily tissue function, and triiodothyronine affects the way the body uses energy, says WebMD. The thyroid gland produces hormones relating to the body's metabolism. Blood testing is required to make sure the thyroid is functioning properly, claims MedicineNet. Thyroxine and triiodothyronine account for over 99 percent of thyroid hormones. The body converts the hormone thyroxine into triiodothyronine. Hormones are needed for proper brain growth and development.