What does the hormone prolactin primarily regulate?

Prolactin is primarily known for its crucial role in regulating milk secretion, particularly during and after pregnancy. This hormone is produced by the anterior pituitary gland and is essential for the development of mammary glands and the initiation of milk production postpartum. High levels of prolactin stimulate the mammary glands to produce milk, making it a key factor in lactation.

During pregnancy, prolactin levels increase, preparing the breast tissue for milk production. After childbirth, the suckling of an infant triggers further release of prolactin to maintain milk supply. This regulation supports the nutritional needs of the newborn, reinforcing the importance of prolactin in breastfeeding and maternal-infant bonding.

In contrast, other hormones have specific functions related to the other options. While menstrual cycles are primarily regulated by hormones such as estrogen and progesterone, bone density is influenced by hormones like estrogen and parathyroid hormone. Similarly, endometrial thickness is affected mainly by estrogen and progesterone during the menstrual cycle. Therefore, the distinctive role of prolactin in milk secretion makes it the correct answer to the question.