Which hormone is primarily responsible for regulating the metabolic rate in animals?

Thyroxine, a hormone produced by the thyroid gland, plays a crucial role in regulating the metabolic rate in animals. It influences various physiological processes, including how quickly the body utilizes energy, synthesizes proteins, and metabolizes carbohydrates and fats. Higher levels of thyroxine increase overall metabolic activity, leading to an increase in energy expenditure. This hormone essentially acts like a metabolic catalyst, enhancing the rate at which cells perform their functions.

In contrast, insulin primarily regulates blood sugar levels, testosterone is involved in the development of male secondary sexual characteristics and may influence muscle mass and strength but does not directly impact metabolic rate, and cortisol, while associated with stress response and metabolism, primarily helps regulate energy during times of stress rather than being the primary regulator of metabolic rate. Thus, thyroxine is the most significant hormone among the options for its role in metabolic regulation.