What is the primary function of prolactin?

Prolactin is a hormone primarily responsible for stimulating milk production in the mammary glands after childbirth. It plays a crucial role in lactation, ensuring that nursing mothers can produce the milk necessary for their infants' nutrition. The secretion of prolactin is regulated by numerous factors, including the hormonal interplay during pregnancy and the act of breastfeeding itself, which enhances its levels to promote milk synthesis and secretion.

In contrast, the other functions mentioned relate to different hormones and biological processes. For instance, stimulating egg production is primarily the role of follicle-stimulating hormone, while testosterone production is influenced mainly by luteinizing hormone and other factors. Growth hormone supports overall growth and metabolism but is not associated with prolactin's direct function. Thus, among the provided options, promoting milk production is the core and defining role of prolactin.