A thyroxine test is used to help diagnose disorders of the thyroid. The thyroid is the butterfly-shaped gland at the base of your neck. It produces hormones that regulate your body's energy use, along with many other essential functions. As part of the endocrine system, the thyroid gland makes and stores hormones that help regulate the body's metabolism in the form of blood pressure, blood temperature, and heart rate.
Thyroxine (T4) is the primary hormone that is produced and secreted by the thyroid gland. It is responsible for digestion, heart rate, muscle formation, brain development, and healthy bones. T4 is biologically inactive (called a storage hormone); its primary function is to transport the hormone triiodothyronine (T3) to the proper organs in your body. Without enough T4 (and its T3 counterpart), the body's metabolism slows down.
Too much or too little T4 may indicate thyroid disease.
Thyroxine exists in two forms in the body: Free T4 and Bound T4. While most of these hormones are bound to protein and inactive, the "free" active component helps influence metabolism. Normal levels of Free T4 are essential for normal metabolism and temperature regulation. Free T4 does not bond to the protein in your blood, which allows it to enter the body tissues that need to use it. Most of the T4 in the bloodstream, however, is bonded to protein, which prevents it from entering these tissues; this is Bound T4.
While you can test both forms of T4 with a Total T4 test, a Free T4 test is considered to be more accurate and helpful because T4 converts into Triiodothyronine (T3), another thyroid hormone. Any changes show up in T4 first.
A thyroxine test can help you, and your doctor understand how your thyroid is working or if there may be a need for further evaluation.
You should stop taking biotin, a vitamin commonly found in many hair, nail, and skin supplements, 12 hours before your test. Otherwise, continue your medication and diet as usual. If you have any questions about a specific drug, you should consult with your physician before the test.
Conventionally, a health care professional will collect a blood sample from a vein in your arm, using a small needle, and sends it to a lab for testing.
Collecting your sample should take no more than 5 minutes. You will then send it to a CLIA-certified lab for analysis.
Test results can vary depending on your age, gender, history, laboratory methods, and other variables.
T4 isn't the only hormone participating in thyroid function, so an abnormal result on this test alone may not provide enough information to understand or diagnose your condition. Looking at TSH, Free T4, Free T3, and TPO antibodies together can give a complete picture.
High levels of T4 may indicate hyperthyroidism (overactive thyroid) or Grave's disease (autoimmune thyroiditis).
Low levels of T4 may indicate hypothyroidism (underactive thyroid) or Hashimoto's Disease (autoimmune thyroiditis).
To note: if you're pregnant or planning to be, you should discuss your values with a doctor as normal ranges may be different. Both Total T4 and Free T4 vary in children; you should talk to your doctor about expected ranges for your child.
Talk with a doctor you trust about what the test results may mean about your thyroid function.
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