Thyroid Hormones Increase Cellular Metabolic Activity
The thyroid hormones increase the metabolic activities of almost all the tissues of the body. The basal metabolic rate can increase to 60 to 100 per cent above normal when large quantities of the hormones are secreted. The rate of utilization of foods for energy is greatly accelerated. Although the rate of protein syn-thesis is increased, at the same time the rate of protein catabolism is also increased. The growth rate of young people is greatly accelerated. The mental processes are excited, and the activities of most of the other endocrine glands are increased.
Thyroid Hormones Increase the Number and Activity of Mito-chondria. When thyroxine or triiodothyronine is givento an animal, the mitochondria in most cells of the animal’s body increase in size as well as number. Furthermore, the total membrane surface area of the mitochondria increases almost directly in proportion to the increased metabolic rate of the whole animal. Therefore, one of the principal functions of thyroxine might be simply to increase the number and activity of mitochondria, which in turn increases the rate of formation of adenosine triphosphate (ATP) to ener-gize cellular function. However, the increase in the number and activity of mitochondria could be the result of increased activity of the cells as well asthe cause of the increase.
Thyroid Hormones Increase Active Transport of Ions Through Cell Membranes. One of the enzymes that increases itsactivity in response to thyroid hormone is Na+-K+-ATPase. This in turn increases the rate oftransport of both sodium and potassium ions through the cell membranes of some tissues. Because this process uses energy and increases the amount of heat produced in the body, it has been suggested that this might be one of the mechanisms by which thyroid hormone increases the body’s metabolic rate. In fact, thyroid hormone also causes the cell membranes of most cells to become leaky to sodium ions, which further activates the sodium pump and further increases heat production.