T4, or thyroxine, is one of the two main hormones produced by the thyroid gland. Its release is regulated by TSH, or thyroid stimulating hormone. T4 is a prohormone, meaning it is a precursor to T3, and when released it is converted into T3.
T4 helps your body with for example regulation of metabolism, body temperature and heart rate. T4 is also known for supporting brain development and is a relatively common cause for unexplained fatigue.
Normal T4 levels indicate that you have a healthy thyroid gland and a balance in the regulatory system controlling the release of thyroid hormones.
Playing an important role in many crucial regulatory functions of our body, measuring T4 is of interest when primarily evaluating the health and function of the thyroid gland.
Additionally, since the release of T4 is directed by the hypothalamus and pituitary gland in the brain, measuring T4 can help in understanding whether or not these structures are healthy and well functioning. Thirdly, measurements of T4 are valued when evaluating and tracking treatment effectiveness with thyroid hormone replacement therapy.
High T4 levels, also known as hyperthyroidism, mean that your thyroid gland is producing too much thyroxine hormone. The most common causes behind this are conditions in the gland, such as inflammation (thyroiditis), and an autoimmune disorder resulting in an overproduction of thyroid hormones (Grave’s disease). Excessive intake of thyroid hormones through replacement therapy as well as excessive intake of iodine may also result in hyperthyroidism.
Low T4 levels, also known as hypothyroidism, mean that your thyroid gland is not producing enough thyroxine hormone. The most common causes behind this are conditions in the gland, such as inflammation (thyroiditis), and treatment therapies such as surgery and radiation therapy. Iodine deficiency, pituitary gland dysfunction and certain medications such as lithium can also cause hypothyroidism.