Thyroxine (T4) Test
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| Thyroxine (T4) Test : What is a thyroxine (T4) test? Thyroxine is also called T4. T4 is a hormone that your thyroid gland makes. A T4 test measures the level of T4 in a sample of your blood. Too much or too little T4 may be a sign of a thyroid problem. Your thyroid is a small, butterfly-shaped gland in the front of your neck. It makes hormones that control the way your body uses energy. These hormones affect nearly every organ in your body and control many of your body's most important functions. For example, they affect your breathing, heart rate, weight, digestion, and mood. In children, thyroid hormones affect growth, too. T4 is the main hormone that your thyroid makes. There are two forms of it in your blood: - Free T4 is the active form of thyroxine hormone that enters your body tissues where you need it. - Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your body tissues. It stays in your bloodstream as a "backup supply" until your tissues need it. T4 levels can be measured with either a free T4 test or a total T4 test: - A free T4 test measures the amount of free T4 in your blood. Medical experts believe this test is more accurate than a total T4 test, so it's used more often. - A total T4 test measures free and bound T4 together. A T4 test alone can't provide enough information to diagnose thyroid problems. So, it's usually done with a TSH blood test. TSH stands for thyroid stimulating hormone. It's a hormone made by your pituitary gland, a small gland at the base of your brain. TSH tells your thyroid how much hormone to make. Normally, if your T4 levels are too low, your pituitary makes more TSH to make your thyroid work harder. If your T4 levels are too high, your pituitary stops making TSH. Other names: free thyroxine, free T4, total T4 concentration, Free T4 index (FTI), thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis | |
| Review Date: 12/12/2024 | Updated By: |