The hormone that primarily facilitates glycogen formation in the liver while simultaneously suppressing gluconeogenesis is Insulin.

- Insulin is an anabolic hormone secreted by the pancreatic beta cells in response to elevated blood glucose levels, such as after a meal.
- It promotes the uptake of glucose by liver cells and stimulates the enzyme glycogen synthase, which enhances glycogen synthesis (glycogenesis).
- At the same time, insulin inhibits key enzymes involved in gluconeogenesis, such as phosphoenolpyruvate carboxykinase (PEPCK), thereby reducing glucose production in the liver.
- This dual effect helps to lower blood glucose levels and store energy efficiently.

- In contrast, hormones like glucagon and epinephrine stimulate glycogen breakdown (glycogenolysis) and gluconeogenesis to increase blood glucose during fasting or stress.
- Cortisol also promotes gluconeogenesis rather than suppressing it. Therefore, only insulin encourages glycogen formation and limits glucose production simultaneously.

Key points:
- Insulin promotes glycogen synthesis in the liver.
- Insulin suppresses gluconeogenesis, decreasing glucose output.
- Glucagon, epinephrine, and cortisol have opposing effects, increasing gluconeogenesis.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 73 - Metabolic Regulation of Glucose