Which hormone counteracts the effects of insulin?

Glucagon is a hormone that plays a crucial role in glucose metabolism by opposing the effects of insulin. While insulin works to decrease blood glucose levels by promoting the uptake of glucose into tissues and stimulating the liver to store glucose as glycogen, glucagon serves to increase blood glucose levels when they are low. It does this primarily by stimulating the liver to convert glycogen back into glucose (glycogenolysis) and by promoting the synthesis of glucose from non-carbohydrate sources (gluconeogenesis).

In situations of low blood sugar, such as fasting or prolonged physical activity, glucagon is released from the alpha cells of the pancreas to ensure that the body has sufficient glucose for energy, particularly for vital organs like the brain.

Although cortisol and adrenaline also have roles in glucose metabolism and can have effects that counterbalance insulin, glucagon is the most direct hormone in this function regarding blood glucose regulation. Therefore, while multiple hormones can influence blood sugar levels, the primary and most direct antagonist to insulin's effects in raising blood glucose is glucagon.