T4, or thyroxine, is a hormone produced by the thyroid gland that plays a crucial role in regulating metabolism in the body. It helps control how the body uses energy, affecting processes such as heart rate, digestion, and temperature regulation. T4 is essential for growth and development, particularly in children. In the bloodstream, T4 is often converted into another hormone called triiodothyronine (T3), which is more active and potent. The levels of T4 in the body are regulated by the pituitary gland through a feedback system involving thyroid-stimulating hormone (TSH). Proper T4 levels are vital for overall health and well-being.