What Is the Purpose of the Hormone Prolactin?

What is prolactin?

Prolactin or luteotropic hormone (LTH) is a peptide hormone, which promotes milk secretion in female mammals. This hormone also plays a significant role in pancreatic development, immune system regulation, and metabolism. When women become pregnant, their prolactin levels increase by 20 fold. However, the role or prolactin is so much more than promoting lactation since it also influences over 300 processes in the body, which include water and salt regulation, reproductive behavior, and metabolism, among others. 

Normal Levels of Prolactin

The normal values for prolactin may slightly vary among laboratories. Usually, the results are shown in nanograms per milliliter (ng/mL). Below are normal values for prolactin:

• Female: less than 25 ng/mL

• Pregnant females: 34 to 386 ng/mL

• Male: less than 15 ng/mL

Abnormal Prolactin Levels

Normally, healthy males and nonpregnant females only have traces of prolactin in the blood. However, when individuals have hyperprolactinemia or an increased prolactin level, it may be a sign of an underlying medical condition. Hyperprolactinemia occurs in approximately 10 percent of the population. Some of the causes of hyperprolactinemia are hypothyroidism, pituitary tumors, kidney disease, liver problems, and even anorexia nervosa. However, during pregnancy and after giving birth, high prolactin levels are normally expected in women.

High prolactin levels can also be caused by certain drugs, which include psychiatric medications such as haloperidol (Haldol) and risperidone (Risperdal). Taking medications for stomach problems (acid reflux or nausea) like metoclopramide can also lead to increased levels of prolactin.

Other factors that can significantly increase prolactin levels are hypoglycemia (low blood sugar), mild discomfort, and vigorous exercises or strenuous activities. 

Prolactin Regulation and Secretion

Dopamine is a hormone that is produced in the brain’s hypothalamus. This hormone regulates prolactin production from the pituitary gland. Dopamine restricts the production of prolactin, so when there is more dopamine produced, the lesser amount of prolactin is released. However, prolactin enhances dopamine secretion, which creates a negative feedback.

Another main prolactin regulator is the hormone called estrogen. This hormone increases prolactin secretion and production from the pituitary gland. According to studies, there is a small increase in prolactin levels during the reproductive cycle of women, wherein estrogen levels are at peak. The same thing happens during pregnancy and after delivery since increased prolactin levels are needed to initiate lactation.

Aside from the hormones estrogen and dopamine, other hormones can also influence the levels of prolactin in the body, and they include oxytocin, thyrotropin-releasing hormone, and antidiuretic hormone.

Functions of Prolactin

1. Breast milk production

The production of breast milk occurs when high prolactin levels are maintained as well as having an abrupt drop in the level of progesterone and estrogen. After childbirth, both progesterone and estrogen levels rapidly drop until women start ovulating again. Estradiol then acts to increase the secretion of prolactin at the hypothalamic level. To stimulate genes that encode milk proteins, cortisol along with insulin act together during the process of milk production.

The uptake of both amino acids and glucose is stimulated. It also stimulates the synthesis of fats, sugar, milk proteins, phospholipids, and spermidine. To maintain milk production, nonpregnant levels of prolactin are sufficient. In mothers who are breastfeeding, a suckling infant stimulates the secretion and production of prolactin.

In and around the nipples, the mechanoreceptors are activated by suckling and the nerve fibers carry the signal to the hypothalamus through the spinal cord. To export calcium to the breast milk, bone loss can happen due to high prolactin levels.

2. Breast development

Hormones and growth factors support breast development. The hormones usually required during the process of breast development are prolactin, progesterone, estrogen, insulin, growth hormone, thyroid, cortisol, and parathyroid hormone. All these hormones work together to develop a functional gland. During pregnancy, if the prolactin levels are high, the mammary glands enlarge to prepare the body for milk production.

3. Sexual desire and nurturing behavior

High levels of prolactin are connected to nurturing human qualities, parenting behavior, psychosomatic reactions, and even phantom pregnancy (false pregnancy). Prolactin may have a vital role to play when it comes to paternal care. This effect is even seen in animals, such as birds and fish. According to some studies, an association has been found between paternal care and prolactin levels in males that rear their offspring.

Prolactin levels in women can increase or decrease sexual drive depending on a variety of factors. However, the sexual behavior of male rats is suppressed by prolactin.

4. Pregnancy and childbirth

Prolactin is also referred to as the mothering hormone. The role of prolactin during childbirth has not been researched much as compared to other hormones. During pregnancy, the prolactin level increases, and when labor starts, the prolactin levels peak as it is seen in other mammals.

A woman’s body becomes ready for breastfeeding since prolactin is constantly produced during pregnancy and after childbirth. Moreover, prolactin may play a role in regulating labor as well as help newborn babies adjust to the outside world. High prolactin levels also promote women’s caretaking behavior and early breastfeeding. 

5. Fetal brain development 

The hormone prolactin also stimulates fetal development, growth, and metabolism along with stimulating myelin formation on axons (nerve fibers) in the brain. It also encourages neurogenesis in maternal and fetal brain, fetal immune tolerance during gestation, and fetal lung development. 

6. Infant breathing

As previously mentioned, prolactin plays a very important role in fetal development. It allows the diffusion and absorption of surfactant molecules at interfaces between water and air at birth, as the newborn uses its lungs outside its mother's womb. Prolactin levels help the baby's immune tolerance and immune system development.

7. Sexual gratification

According to a recent research in sexual gratification, prolactin may play a role after sexual intercourse. The hormone responsible for sexual drive and arousal is dopamine. For sexual arousal, the response of dopamine is repressed by prolactin levels. Degrees of sexual satisfaction can be indicated by the levels of prolactin after sexual intercourse.  High levels of prolactin have been associated with libido loss, suppression of ovulatory cycle, and impotence.

8. Stimulates brain cell proliferation

Precursor cells could differentiate later into organs or they could be multipotent stem cells. The proliferation of precursor cells is stimulated by prolactin functions. One such example is the proliferation of oligodendrocyte precursor cells for CNS function.

9. Fertility

High levels of progesterone help sustain pregnancies. However, abnormally elevated levels of this hormone are also associated with spontaneous abortion, which is why women who breastfeed are often protected against premature pregnancy due to low fertility. 

The Role of Prolactin in Diagnosis

Usually, prolactin levels are used to check for the following conditions:

• Normal function of sex hormones
• Check for polycystic ovarian syndrome and ovarian cycle
• Hypogonadism, impotence, and erectile dysfunction in men
• Breastfeeding and galactorrhea in women
• Function of dopamine
• Hyperprolactinemia
• Epileptic seizures versus nonepileptic seizures
• Tumor of the pituitary gland or prolactinoma
• Kidney disease 
• Vision problems
• Headaches
• Medically induced prolactin deficiency due to antipsychotic medications
• Eating disorders (anorexia or bulimia)

Other Hormones Affected by Prolactin

When there is an elevated level of prolactin, the levels of estrogen in females and testosterone in males decrease. Such changes in prolactin levels are greater in females, wherein there is a substantial increase or decrease in estrogen levels. Elevated prolactin levels inhibit GnRH secretion leading to decreased levels of luteinizing hormone and follicle-stimulating hormone. 

Testosterone secretion and sperm cell production in males are enhanced when they have normal prolactin levels. Prolactin can also reduce hepcidin, which mainly regulates iron levels in the blood. Prolactin also leads to the indirect release of parathyroid hormone-related peptide (PTHrP), which is associated with bone loss. 

Low Levels of Prolactin

There are certain problems associated with low levels of prolactin. They include:

• Poorer transition of the baby at the time of birth
• Poor growth and development of the baby
• Poor ability of women to adjust to motherhood

The body’s production of prolactin can be promoted by:

• Waiting for labor to naturally occur
• Minimal stress during labor and after childbirth 
• Keeping the mother and her baby together after birth 
• Early breastfeeding

Prolactin Test

Prolactin levels can be measured with the help of a simple blood test. This test is done at the fertility doctor’s office. Doctors usually request that the test is performed early in the morning to get accurate results. Although prolactin levels tend to vary throughout the day or night, prolactin is usually at peak during sleep and just after waking up, during times of emotional stress, and after intense workouts.

A prolactin test is a safe procedure. You can speak with a doctor if have questions about the test or speak with a technician before the procedure.