Which hormone acts as an antagonist to insulin?

Glucagon is recognized as the primary antagonist to insulin because it plays a crucial role in regulating blood glucose levels. When blood glucose levels drop, glucagon is secreted by the alpha cells of the pancreas. Its primary function is to stimulate the liver to convert stored glycogen into glucose, releasing it into the bloodstream. This process increases blood sugar levels, counteracting the effects of insulin, which primarily facilitates the uptake of glucose into cells and lowers blood sugar levels.

In contrast, cortisol, adrenaline, and thyroxine have hormonal functions that can influence glucose metabolism but do not directly oppose insulin in the same way that glucagon does. Cortisol can promote gluconeogenesis and affect insulin sensitivity, while adrenaline can increase blood glucose levels during stress through glycogenolysis. Thyroxine primarily regulates metabolism and energy expenditure but does not have a direct antagonistic role against insulin like glucagon. Therefore, glucagon is the most distinct and direct antagonist to insulin, making it the correct answer in this context.