What hormone is primarily responsible for milk synthesis?

The hormone primarily responsible for milk synthesis is prolactin. Prolactin is produced by the anterior pituitary gland and plays a crucial role in initiating and maintaining milk production in the mammary glands after childbirth. Its secretion is stimulated by the hormone estrogen during pregnancy, but the primary action of prolactin is to promote the synthesis of milk.

While progesterone and estrogen are important for preparing the breast tissue during pregnancy, and oxytocin is essential for the milk ejection reflex (also known as the let-down reflex), they do not directly stimulate the synthesis of milk. Prolactin's primary function is specifically tied to the production of milk, making it the key hormone for this biological process.