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Thyroxine (T4) is the main hormone your thyroid makes. Your thyroid hormones are important because they control how your body uses energy from the food you eat. (Photo Credit: iStock/Getty Images) T4 ...

A complement C4 (also known as a complement component 4, or simply C4) is a test to measure the level of C4 proteins you have in your blood. These proteins play a role in how your immune system ...

Jay N. Yepuri, MD, MS, FACG, is a board-certified gastroenterologist and member of the Digestive Health Associates of Texas Board of Directors and Executive Committee. A liver function test checks how ...