The Multifaceted Role of Prolactin in Human Physiology

Prolactin (PRL) is a hormone with a critical role in lactation, especially following childbirth. After delivery, levels of maternal hormones such as estrogen and progesterone decline, and in the presence of cortisol, prolactin initiates and maintains milk production. This process is finely tuned; if prolactin levels drop, milk secretion ceases immediately. Additionally, prolactin inhibits the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary, leading to a natural form of contraception known as physiological secondary amenorrhea during the postpartum period.

Interestingly, prolactin is not exclusive to mammals. In birds, this hormone promotes behaviors like nest-building and the production of crop milk. It also plays diverse roles in reptiles, amphibians, and certain fish, where it acts as an osmoregulator. The conservation of prolactin-like molecules across different species suggests that this hormone may have various functions beyond those currently understood, particularly in human males and females. However, the physiological significance of many of these proposed functions remains an area of ongoing research.

The regulation of prolactin mirrors that of growth hormone (GH). Prolactin is primarily under the tonic inhibition of dopamine, with thyrotropin-releasing hormone (TRH) providing a stimulatory effect. Stress can cause an increase in serum prolactin levels, and while its release is not as episodic as that of GH, there are peaks during sleep. During pregnancy, prolactin levels can increase up to tenfold, largely driven by rising estrogen levels. This increase is sustained during lactation due to suckling, forming a positive feedback loop: the act of suckling stimulates prolactin release, enhancing milk production, which in turn promotes further suckling.

However, hyperprolactinemia, a condition characterized by elevated prolactin levels, can lead to several clinical symptoms, including oligomenorrhea or secondary amenorrhea and sub-fertility in women of reproductive age. This occurs as high prolactin levels inhibit the pulsatile secretion of LH and FSH, disrupting normal ovarian function. In some cases, inappropriate breast milk production, also known as galactorrhea, can occur. The most common underlying cause of hyperprolactinemia is a microprolactinoma, a benign tumor of the pituitary gland that secretes excess prolactin.

Given the prevalence of hyperprolactinemia, it is a condition that healthcare providers should be aware of, especially when evaluating reproductive health issues. Understanding the functions and regulatory mechanisms of prolactin is essential for comprehending its impact not only on lactation but also on broader reproductive and physiological processes in both women and men.