21.12: Functions of Thyroid Hormones
The thyroid hormone (TH) plays a pivotal role in the intricate orchestration of physiological processes, exerting profound effects on development, metabolism, and homeostasis throughout different life stages.
TH is indispensable for the normal development and maturation of the skeletal, muscular, and nervous systems during fetal and childhood growth. It facilitates bone mineral turnover and regulates protein synthesis in developing tissues, contributing significantly to overall growth and maturation.
TH continues its influence upon reaching adulthood, particularly on the reproductive and circulatory systems. Primarily in females, it regulates gonadal hormone secretion, impacting the reproductive system, and stimulates endocrine tissue, contributing to hormonal balance. In the circulatory system, TH promotes normal heart function, facilitates red blood cell synthesis, and enhances tissue oxygenation.
TH binds to various receptors within the cell, including cytoplasmic, nuclear, and mitochondrial receptors. Cytoplasmic receptors store TH and release it when intracellular TH levels decrease, maintaining cellular homeostasis. TH activates glycolysis and ATP production genes in the mitochondria, increasing energy synthesis. Simultaneously, TH indirectly enhances ATP production in the nucleus by controlling the synthesis rate of enzymes involved in the cellular metabolic rate.
The culmination of these actions is known as the calorigenic effect, reflecting an elevated cellular metabolic rate induced by thyroid hormone. This multifaceted role underscores the critical contribution of TH to fundamental physiological processes, ensuring the harmonious functioning of various organ systems and the maintenance of overall cellular homeostasis.
