Which gland is mainly responsible for the secretion of glucagon?

The pancreas plays a crucial role in the regulation of blood glucose levels through the secretion of hormones, including glucagon. Glucagon is produced by the alpha cells of the islets of Langerhans in the pancreas and is responsible for raising blood glucose levels when they are too low. It does this by stimulating the liver to convert glycogen into glucose and release it into the bloodstream.

The other glands listed play different roles in the endocrine system. The pituitary gland primarily regulates other endocrine glands and is not directly involved in glucagon secretion. The thyroid gland is responsible for producing hormones that regulate metabolism and does not produce glucagon. The adrenal gland, while involved in stress responses and the production of hormones like adrenaline and cortisol, does not secrete glucagon either.

Thus, the pancreas is uniquely suited to produce glucagon, making it the correct answer.