Which hormone is primarily responsible for regulating metabolism?

Thyroxine, also known as T4, is a hormone produced by the thyroid gland and plays a crucial role in regulating metabolism. It influences the metabolic rate of almost all the body's cells, promoting the conversion of nutrients into energy. This hormone impacts processes such as growth, development, and temperature regulation by increasing oxygen consumption and heat production in tissues, thereby stimulating metabolic activities.

Insulin, while important for regulating blood sugar levels and facilitating glucose metabolism, does not primarily regulate overall metabolism in the way that thyroxine does. Cortisol is involved in the body's stress response, affecting glucose metabolism and immune response, but its main function is not metabolic regulation. Adrenaline, or epinephrine, is primarily associated with the body's fight-or-flight response and quickly mobilizes energy but is not directly responsible for long-term metabolic regulation like thyroxine.

Thus, thyroxine is recognized as the key hormone for metabolism, making it the correct answer.