Thyroxine

-       Iodine-containing hormone secreted by thyroid gland

-       Alternate name :T4 (tetraiodothyronine)

-       Natural occurrence: l-thyroxine

Main function: The main function of T4 is that it stimulates various bodily systems. This can range from metabolism to cardiovascular activity and even nervous activity.  T4 is known for being able to increase the basal metabolic rate, which is the energy used up during rest, which in turn creates body heat. It also increases carbohydrate metabolism which generates free glucose or gluconeogenesis. T4 quantities also has influence over lipid metabolism in that more quantities of the T4 hormone results in more fatty acids present in plasma.

            In regards to other bodily systems, T4 has the capacity to influence the level of activity present. In the cardiovascular system, high T4 levels can increase the heart rate increasing blood circulation. In the nervous system, excess T4 results in anxiety while lack of T4 results in slow mental processes.

            T4 is majorly connected to stimulating growth of the human body. T4 is present in the body from fetal development and persists long after. It help perpetuate bone growth and activates other growth hormones. Examples are that T4 activates erythropoietin and somatomedins, hormones that stimulates RBC production and cell growth and division respectively.

Mechanism: T4 is carried into the cell by the carrier protein Thyroxin-Binding Globulin. When the T4 crosses into the cytoplasm, it is converted into T3 (Triiodothyronine). T3 then binds to a nuclear receptor protein which then binds to its respective hormone response sites in DNA which then causes mRNA to form which then produces other proteins.