The Hormone Prolactin
Prolactin is s hormone synthesized and secreted from the lactotroph cells in the anterior pituitary gland. It is also produced by various other tissues in the body but to a far lesser degree than in the anterior pituitary. There are three forms of prolactin in the body which differ by molecular weight.
The hormone is probably best known for its effects in pregnancy and breastfeeding women – prolactin promotes growth of breast alveoli and stimulates breast milk production and secretion. This is opposite to the effect of estrogen and progesterone which suppresses milk production yet the high levels of these female hormones in pregnancy promote prolactin secretion.
Despite the drop in estrogen levels after childbirth, the secretion of prolactin is maintained to promote breast milk production for feeding – this is influenced by suckling. The levels of prolactin are also controlled by the hypothalamus which secretes prolactin inhibitory factor – this hormone acts on the anterior pituitary to suppress prolactin release into the bloodstream. Other hormones also play a significant role in prolactin secretion – dopamine suppresses prolactin secretion while thyrotropin-releasing hormone (TRH) and vasoactive intestinal peptide (VIP) promote prolactin secretion.
What is hyperprolactinemia?
Hyperprolactinemia is the medical term for high levels of prolactin in the bloodstream. It can occur in both women and men. Prolactin will lead to breast milk production and this is known as galactorrhea (breast milk secretion when not pregnant). Galactorrhea is a more likely consequence of hyperprolactinemia in women and rarely occurs in men.
Effects of High Prolactin Levels on Men and Women
In hyperprolactinemia, both men and women will report breast enlargement due to the action of prolactin.This is known as gynecomastia in men. Apart from the effect of prolactin on the breast tissue, it can also suppress another important hormone in the body – gonadotropin-releasing hormone (GnRH).
This hormone in turn controls the secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the pituitary gland. LH stimulates the secretion of the androgens (male sex hormones) and estrogens (one group of the female sex hormones) while FSH stimulates growth of the gonads (sex organs) in both males and females. Therefore prolactin indirectly impacts on sexual function and reproduction in both genders. This leads to hypogonadism which is the underactivity of the sex organs (gonads) in males and females to produce the relevant sex hormones.
Causes of High Prolactin Levels
Hyperprolactinemia may be due to physiological or pathological causes. Physiological hyperprolactinemia is due to some change in the body that is not associated with any disease. Some of these physiological factors include :
- Lactation – breastfeeding
- Nipple stimulation
- Sound of a crying baby (postpartum period)
- Sexual intercourse (some women)
Pathological causes of hyperprolactinemia is related to disease process in the body either within the pituitary gland or elsewhere.
- Prolactin-secreting pituitary tumors (prolactinoma). Pre-menopausal women tend to have a microadenoma (tumor < 10mm in diameter) while post-menopausal women and men usually have a macroadenoma (tumor >10mm in diameter).
- Pituitary compression by non-secreting tumors.
- Growth hormone-secreting tumors
These pathogenesis associated with these causes are obvious – pressure on the pituitary stalk increases prolactin secretion while in prolactinoma the tumor itself produces and secretes prolactin.
The different types of drugs that may cause hyperprolactinemia can be divided into three categories – those that block dopamine, those that deplete dopamine or estrogen. Dopamine suppresses prolactin secretion from the pituitary gland so if dopamine is blocked increases prolactin levels. Estrogen stimulates prolactin secretion as is the case in pregnancy. Various other drugs may also cause hyperprolactinemia through different pathways like increasing factors that stimulate prolactin secretion (example VIP and TRH).
- Dopamine blockers
- Dopamine-depleting drugs
- Oral contraceptives
The hypothalamus plays an important role in regulating prolactin secretion and can both promote or suppress it. Hypothalamic lesions may diminish those hormones that inhibit prolactin or enhance the hormones that promote prolactin secretion.
- Tumors of the hypothalamus
- Infiltrative hypothalamic lesions caused by certain infections and autoimmune disease
- Post infection (encephalitis)
- Head trauma
Various endocrine diseases may cause prolactinemia by directly promoting its secretion or indirectly by hampering those factors that regulate and diminish prolactin levels. It may also contribute to one or more factors that are known to stimulate prolactin secretion.
- Primary hypothyroidism (high thyrotropin-releasing hormone levels)
- Cushing’s syndrome (high cortisol levels)
- Acromegaly (high growth hormone levels)
Chest Wall Reflex
Any irritation to the chest wall may trigger the same reflexes as nipple stimulation and thereby increase prolactin levels.
- Infections like herpes zoster and may also occur post-infection.
- Trauma including surgery
- Surgical scars
- Polycystic ovarian syndrome (PCOS)
- Renal failure (kidney)
- Liver disease
Signs and Symptoms of High Prolactin Levels
- Abnormal or spontaneous lactation (breast milk secretion)
- Menstrual irregularity in women – secondary amenorrhea (no period) or oligomenorrhea (scanty or infrequent menses)
- Enlarged / engorged breasts in men and women
- Infertility (men and women)
- Diminished libido
- Erectile dysfunction (men)
- Low sperm count (men)
- Visual disturbances