Total T4 (Thyroxine)

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.

If you had a free T4 test, your test results may be reported as "free T4." If you had a total T4 test, your results may be reported as "free T4 index (FTI)." FTI is the amount of free T4 in your blood based on a calculation using your total T4 test results. To fully understand your results, your provider will usually need to compare them with the results of other thyroid tests. So, it's best to talk with your provider about what your results mean.

T4 results that are lower than normal may indicate hypothyroidism, Hashimoto's disease (an autoimmune disorder), certain stages of thyroiditis, pituitary problems, dietary iodine imbalance, or congenital hypothyroidism. Low T4 levels may also result from treatment for hyperthyroidism or thyroid cancer.

T4 results that are higher than normal may indicate hyperthyroidism, Graves' disease, certain stages of thyroiditis, toxic goiter, toxic thyroid nodule, excessive iodine intake, pituitary tumors, or overdose of thyroid hormone medicine.

If your T4 results are abnormal, you may need more testing to find the cause. But abnormal T4 levels don't always mean you have a medical condition that needs treatment. Many things can affect your T4 levels, including severe illness and certain medicines, such as birth control pills and steroids.