Thyroxine is one of the main hormones released by the thyroid gland. The thyroid gland is located in the neck and is regulated by factors released from the brain. Thyroid hormones such as thyroxine are necessary for proper development and regulation of multiple body systems. Excessive or decreased levels can cause problems because of the strong effects of thyroxine on the cells of target tissues.
Thyroid hormones enter the cell through the protective outside layer of the cell called the cell membrane. The hormones bind to protein receptors, which are then taken into the cell nucleus, where the DNA is stored. The hormone-receptor complex changes the expression of DNA in the cell. Some of these changes increase oxygen consumption. Thyroxine increases the use of oxygen by the cells of nearly all tissues in the body. It also increases the use of fatty acids, protein and protein channels in the cell. If extra food is not consumed, the body will use stored fat, vitamins and protein in muscle. Weight loss and vitamin deficiencies will result.
Thyroxine stimulates the cells of the nervous system. Excess thyroid hormone causes decreased reaction time of the reflexes, irritability, rapid thoughts and restlessness. Inadequate levels of thyroxine have the opposite effect, the reaction time of the reflexes is increased and thinking is slow. In addition, protein builds up in the cerebrospinal fluid, which is the fluid that bathes the brain and spinal cord. A lack of thyroid hormone during development causes mental retardation, trouble moving, mutism and deafness.
Thyroid hormones enter the muscle cells and is taken to the nucleus where it has several functions. It increases the expression of proteins responsible for increasing the heart rate and force. This decreases the circulation time. In addition, the blood vessels dilate and the kidneys retain more fluids, making the blood volume in the body larger. Thyroxine also increases the response of the heart to fight or flight hormones such as epinephrine, also known as adrenalin.
Effects on Fat and Cholesterol
Thyroxine stimulates the breakdown of fat by cells of the body. It also lowers cholesterol levels by causing the liver to take up and use more cholesterol from the blood.
Since the levels of thyroxine correlate with the speed of metabolism, or how much oxygen is used, thyroxine stimulates the utilization of energy. In the gastrointestinal tract, the effect is to stimulate the uptake of food for use by the body.
Thyroxine stimulates the muscles to break down protein. In young bones, thyroxine promotes normal growth and development. In adults, it accelerates bone turnover, which is the breakdown and rebuilding of bone.