What hormone is primarily responsible for regulating metabolism?

Thyroxine, also known as T4, plays a crucial role in regulating metabolism in the body. It is produced by the thyroid gland and significantly influences how the body utilizes energy. Thyroxine impacts various metabolic processes, including how the body converts nutrients from food into energy and how it regulates basal metabolic rate (BMR).

In particular, T4 affects almost every cell in the body, promoting the metabolic activities that generate energy and heat. By increasing protein synthesis and stimulating the breakdown of fats and carbohydrates, T4 ensures that the body's metabolic processes run efficiently. Furthermore, it plays a role in growth and development, particularly in the nervous system and during infancy and childhood.

Additionally, while other hormones such as insulin, adrenaline, and cortisol also have roles in metabolism, they do not primarily regulate it. Insulin is central to blood sugar control, adrenaline is more associated with the fight-or-flight response, and cortisol primarily regulates stress responses as well as glucose metabolism but does not maintain overall metabolic regulation like thyroxine does. Therefore, the centrality of T4 in controlling and modulating the body's metabolic rate and energy utilization makes it the correct answer to the question about the hormone primarily responsible for regulating metabolism.