Which hormone plays a critical role in regulating metabolism?

Thyroxine is indeed the hormone that plays a critical role in regulating metabolism. It is produced by the thyroid gland and is essential for the regulation of metabolic processes in the body. Thyroxine influences how the body utilizes energy, affects the rate of metabolism, helps in the synthesis of proteins, and promotes overall growth and development. Elevated levels of thyroxine can accelerate metabolism, while lower levels can slow it down, which is crucial for maintaining energy balance and body weight.

Adrenaline, while involved in the body's fight-or-flight responses, primarily affects cardiovascular function and energy mobilization rather than directly regulating metabolism. Testosterone is mainly associated with the development of male characteristics and reproductive functions, although it does have some effects on muscle metabolism. Cortisol, known as the stress hormone, influences metabolism but more so in terms of how the body responds to stress and manages blood sugar levels rather than being a primary regulator of metabolic rate like thyroxine. Thus, thyroxine's direct influence on metabolic regulation makes it the correct choice.