Which hormone is primarily responsible for stimulating the synthesis of milk?

Prolactin is the hormone that plays a key role in stimulating the synthesis of milk in the mammary glands. It is produced by the anterior pituitary gland and its levels increase significantly during pregnancy and after childbirth, preparing the breast tissue for lactation. Prolactin not only encourages the growth of mammary glands but also directly promotes the production of milk following delivery.

This hormone's action is crucial during the early stages of breastfeeding, facilitating the establishment and maintenance of milk supply in response to the infant's needs. By binding to specific receptors in the mammary glands, prolactin induces the transcription of genes necessary for milk protein production, thereby ensuring that nursing infants receive the nourishment they require.

In contrast, other hormones listed, such as aldosterone, estrogen, and progesterone, play various roles in the body but are not directly responsible for milk synthesis. Aldosterone primarily regulates sodium and potassium levels and influences blood pressure. Estrogen and progesterone are involved in the menstrual cycle and pregnancy, contributing to changes in breast tissue but do not directly stimulate milk production.