Which hormone acts antagonistically to insulin?

Glucagon acts antagonistically to insulin, primarily by elevating blood glucose levels when they drop too low. Insulin functions to lower blood glucose levels by promoting the uptake of glucose by cells and stimulating the storage of glucose as glycogen in the liver and muscle. In contrast, glucagon is secreted by the alpha cells of the pancreas when blood glucose levels are low, prompting the liver to break down glycogen into glucose (glycogenolysis) and to generate glucose from non-carbohydrate sources (gluconeogenesis). This action of glucagon effectively raises blood glucose levels, counteracting the effect of insulin.

While cortisol, adrenaline, and thyroxine have roles in metabolic processes and can influence glucose metabolism, their mechanisms do not directly oppose insulin in the same way that glucagon does. Cortisol can promote gluconeogenesis and has a complex relationship with insulin, but it doesn't act as a direct antagonist. Adrenaline (epinephrine) can mobilize glucose in situations of stress or low blood sugar but does not function solely to counteract insulin; it primarily prepares the body for "fight or flight" responses. Thyroxine (T4) primarily regulates metabolism and energy production rather than directly opposing insulin.

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