Which hormone primarily regulates metabolism and energy levels?

Thyroxine, also known as T4, is a key hormone produced by the thyroid gland that primarily regulates metabolism and energy levels in the body. It plays a crucial role in controlling the metabolic rate, influencing how the body uses energy derived from food. Thyroxine increases the basal metabolic rate, facilitating processes such as protein synthesis, lipid metabolism, and carbohydrate utilization. This regulation of metabolism is essential for maintaining body temperature, growth, and overall energy balance.

While other hormones mentioned have important roles in metabolism, they do not primarily regulate metabolic activities in the same direct way that thyroxine does. For instance, thyroid-stimulating hormone (TSH) is involved in stimulating the thyroid gland but does not directly regulate metabolic processes itself. Glucagon mainly works to increase blood glucose levels by promoting the conversion of glycogen into glucose, and cortisol, a steroid hormone, is related to the body's response to stress and can influence metabolism, but its primary roles include regulating metabolism of fats, proteins, and carbohydrates rather than directly managing energy levels like thyroxine. Thus, thyroxine stands out as the main hormone focused on metabolism and energy regulation.