Prolactin does so much more than stimulating milk production – it boosts immunity, combats stress, and shields the brain. In this article, you will learn how prolactin functions in your body and the factors that control its secretion. Read on!
What is Prolactin?
The roles of prolactin in the human body are actually much more complex and not limited to females; it’s an all-around hormone that controls metabolism, immunity, reproduction, mental health, and more. It has over 300 effects in the human body [3, 2, 4, 5].
Where is it Produced?
The majority of prolactin comes from your brain. More precisely, your anterior (front) pituitary gland contains so-called lactotroph cells that create and release prolactin into the bloodstream [1, 3].
- The uterus
- Immune cells
- Skin cells
- Fat tissue
- Boosts lactation and breast development
- Supports pregnancy and fetal development
- Relieves stress and makes you more nurturing
- Stimulates the immune system
Hormones in Control of Prolactin
Prolactin-releasing Peptide (PrRP) reduces appetite and stimulates the pituitary gland to release prolactin. Currently, researchers are exploring PrRP as an anti-obesity therapy .
Dopamine is the primary Prolactin Inhibiting Factor (PIF) and the most important regulator of its secretion. Prolactin boosts dopamine in the hypothalamus, which in turn inhibits further prolactin release – a common mechanism known as a negative feedback loop [3, 10].
- Breast stimulation
- Some drugs (antipsychotics)
Thyrotropin-Release Hormone (TRH)
Next, a spike in thyroid hormones blocks both TRH and TSH production – another negative feedback loop. People with an underactive thyroid or hypothyroidism lack this control mechanism and have increased TRH, which may raise their prolactin [11, 18].
Besides blocking dopamine, estrogen directly boosts the growth of prolactin-producing cells and stimulates their secretion. During pregnancy, prolactin blood levels follow the gradual rise in estrogen [13, 19].
During pregnancy, estrogen and progesterone block prolactin receptors in the breasts and thus prevent lactation. Birth causes a sharp fall in these hormones, which enables milk production and nursing [20, 21].
Oxytocin works hand-in-hand with prolactin during lactation to eject the produced milk. Cells that produce prolactin have oxytocin receptors, and these hormones likely stimulate each other’s production .
- Intimate contact with her baby
- Hearing the baby cry for food
- Loving thoughts about the baby
- Support and comfort from her environment
According to a study on rats, prolactin may inhibit oxytocin in non-pregnant females, but it switches to stimulating effect in pregnancy .
1) Helps Make Breast Milk
Right after delivery, prolactin starts boosting the production of proteins, lactose, and other components of breast milk. Other hormones essential for this process include insulin and oxytocin [4, 35].
Each episode of nursing peaks the prolactin levels and ensures a steady milk supply. If the mother stops nursing, prolactin returns to non-pregnant values after 1-2 weeks .
As mentioned, intimate contact with the baby and a supportive environment stimulate lactation by boosting prolactin and oxytocin.
On the other hand, high prolactin prevents ovulation during breastfeeding (by suppressing GnRH), which protects the mother from a new pregnancy. In fact, women who breastfeed their babies frequently and for long periods of time can naturally delay ovulation for 1-2 years [36, 23].
2) Plays a Role in Breast Development
Milk production and ejection require healthy, functional mammary glands, which form during breast development.
Breast tissue develops in puberty and continues to undergo cyclical changes throughout a woman’s life. The biggest changes happen during pregnancy, when the breasts are thought to fully mature .
Their complex interplay results in the enlargement of mammary glands, preparing them for milk production.
3) Helps Maintain Pregnancy
The other name for prolactin is luteotropic hormone (luteotropin), which indicates another vital role of this hormone. It stimulates the ‘yellow body’ (corpus luteum) in the ovaries to make progesterone, which helps establish and maintain pregnancy .
Additionally, it enables the embryo to attach to the uterus lining and boosts the formation of the placenta.
4) Supports Fetal Growth and Development
Prolactin resembles growth hormone, which is essential in all stages of fetal development .
We are still learning about the exact roles of prolactin in the prenatal stage; scientists suggest it enables brain development and regulates immunity .
Studies on rats and mice have revealed widespread expression of PRL receptors in the fetus, voicing its crucial roles in fetal growth. The placenta also releases compounds similar to prolactin, known as placental lactogens [44, 45].
5) Makes You More Nurturing
During and after pregnancy, prolactin changes the mother’s brain and induces the appropriate behavior in caring for the baby. Higher prolactin levels are associated with nurturing and parenting practices in men and women alike .
According to a study on mice, PRL stimulates the mother’s brain and sharpens her sense of smell, which is essential for the recognition and rearing of the offspring .
However, high prolactin may make new moms hostile, which is probably an adaptive mechanism to protect the young .
6) Combats Stress
Prolactin shields the brain and immune system of new moms against the negative impact of stress .
A study on rats has confirmed the calming effect of prolactin in both sexes, regardless of pregnancy .
7) Boosts the Immune System
This strengthens the response to outside threats such as microbial infections. One animal trial even reported the ability of prolactin to induce an anti-cancer immune response .
8) Regulates Food Intake and Metabolism
Our fatty tissue, liver, and pancreas have prolactin receptors, indicating its role in energy use and intake .
Prolactin increases appetite and food intake by causing leptin resistance. This effect is meant to provide extra energy and nutrients during pregnancy, but it can happen in all conditions with high prolactin [56, 57, 58, 59, 60].
9) Protects the Brain and Nerves
A study on mice revealed a crucial role of prolactin in the hippocampus, the brain’s center for emotions and memory. The hippocampus was up to 80% less developed in mice devoid of PRL, resulting in learning and memory impairments .
10) Maintains Electrolyte Balance
Prolactin maintains the balance under normal conditions but also plays a role in fluid buildup due to liver and kidney diseases .
Although the central roles of prolactin are around pregnancy and nursing, men also produce significant amounts of it. Other functions listed above apply to both sexes and confirm that prolactin is much more than just a “milk” hormone [73, 74].
Prolactin may not be essential for reproduction in men, but its optimal levels enhance testosterone secretion and the production of semen. The overnight increase in testosterone is likely related to a spike in prolactin [3, 76].
In 20 infertile men with low prolactin (hypoprolactinemia), prolactin treatment reduced abnormal sperm cells by 42%. The doctors regarded low prolactin as one of the primary causes of the participants’ infertility .
According to an observational trial with over 2.5K people, low prolactin is associated with :
- Erectile dysfunction
- Premature ejaculation
- Metabolic syndrome
On the other hand, chronically high prolactin (hyperprolactinemia) causes a drop in sex hormones and sexual dysfunction in both men and women. High levels of prolactin will block hormones that are essential for reproductive health (GnRH) [79, 23].
In summary, both high and low prolactin can hinder sexual function and reproductive health in men while normal levels support them.
Can Men Lactate?
Nipple discharge in males (and non-nursing females) is an abnormal process known as galactorrhoea, and the growth of male breast tissue is called gynecomastia. They may occur due to transient hormonal disbalance in puberty .
- Tumors that secrete prolactin (prolactinomas)
- Disrupted sex hormones
- The use of anabolic substances in bodybuilding
- Estrogen-raising drugs
Interestingly enough, nipple discharge of one young man (due to high PRL) had all essential components of mother’s milk. Some scientists have even speculated that men’s “lactation” is an adaptive mechanism meant to replace mother’s milk on extreme occasions [86, 87].