Prolactin: The Third Hormone in Breast Cancer

Prolactin: The Third Hormone in Breast Cancer

Prolactin is a hormone that naturally rises during pregnancy and breastfeeding. It supports the growth of milk-producing glands in the breast and triggers and maintains milk production after birth.

Its primary function is to stimulate breast tissue growth to support breastfeeding—so by nature, it promotes cell proliferation. This is essential during lactation, but when prolactin levels are elevated outside of that context, it can become a red flag.

1. Prolactin is also often referred to as the “third hormone in breast cancer” because it plays a direct role in breast tissue growth and development. 

High levels can stimulate breast cell growth and enhance estrogen receptor activity; intensifying estrogen’s effects. In certain breast cancers, prolactin receptors are overexpressed, making tumor cells more responsive and aggressive in its presence. 

The body’s hormones are like a symphony. An imbalance in one can also cause imbalances in the others. That is especially true when it comes to prolactin, estrogen, progesterone, and thyroid hormones, which when imbalanced can shift the entire hormonal environment in ways that support cancer growth.

2. Elevated prolactin is associated with high estrogen and low progesterone. 

Estrogen stimulates prolactin production by increasing the number and sensitivity of prolactin-producing cells (lactotrophs) in the pituitary gland (the part of the brain where prolactin is secreted).  During pregnancy, this is a normal and necessary process. However, chronically elevated estrogen (often from stress, environmental xenoestrogens, or estrogen dominance) can lead to excess prolactin.

Elevated prolactin can enhance estrogen receptor activity, intensifying estrogen’s effects. 

Progesterone acts as a natural counterbalance. It encourages cells to mature and specialize rather than to continue growth like we see with estrogen and prolactin. It helps calm estrogen’s stimulating effects and supports dopamine, the neurotransmitter that suppresses prolactin. When progesterone is low, there’s less inhibition of prolactin, allowing it to rise more easily.

3. Elevated prolactin is often associated with hypothyroidism. 

This is because when thyroid hormones are low, the brain increases production of TRH (thyrotropin-releasing hormone) to stimulate TSH (thyroid stimulating hormone). TSH tells the thyroid to secrete more thyroid hormones. 

But it also stimulates the release of prolactin at the same time. So when the body is trying to correct low thyroid levels, prolactin can rise as a side effect. Because of this, prolactin levels are actually a good marker of thyroid function. 

Low thyroid function itself has been linked to a higher risk of cancer. Low levels lead to impaired metabolism and cellular repair. When thyroid hormones are low, cells produce less energy, differentiate poorly, and become more vulnerable to stress, all conditions that promote cancer.

4. Elevated prolactin suppresses reproductive hormones and the signals needed for ovulation. 

This is actually a protective mechanism to preserve energy. For example, breastfeeding is an extremely energy-demanding time that requires extra nutrients and resources, so prolactin naturally rises to pause fertility while the body focuses on nourishing the newborn.

This same thing happens in response to low thyroid function. This, too, is the body’s way of conserving energy. When thyroid output is low and cells aren’t producing enough energy, the body prioritizes basic survival over reproduction. High prolactin is often a sign that the body doesn’t have the capacity or reserves needed for the demanding process of creating new life.

However, by suppressing ovulation, prolactin directly lowers progesterone. Since progesterone is only made after ovulation, anything that blocks it leads to a deficiency. Outside of breastfeeding, this can become problematic, because estrogen levels are often left unopposed. Without enough progesterone to balance its effects, estrogen can promote excessive cell growth and raise the risk of developing breast cancer.

Prolactin is interesting because, unlike most hormones that are regulated by signals telling the body when to release them, prolactin is mainly controlled by a signal that tells the body not to release it. That signal is dopamine. In other words, prolactin is secreted by default, unless dopamine is present to hold it back. This makes prolactin one of the few hormones where the body relies on active inhibition, rather than stimulation, to keep its levels low.

Dopamine, released from the hypothalamus, acts as a natural brake on prolactin. When dopamine levels drop, prolactin is free to rise. This system becomes especially important during breastfeeding, when prolactin needs to be elevated. Suckling reduces dopamine temporarily, which lifts the brake and allows the pituitary to release prolactin. Prolactin then stimulates milk production and supports the nurturing state, including maternal behaviors.

Outside of this natural window, however, elevated prolactin can become a problem. In breast cancer, prolactin has been shown to promote cell growth, increase estrogen receptor activity, and support tumor progression—especially in cancers that overexpress prolactin receptors. Elevated prolactin not only encourages proliferation but also interferes with the balance of other protective hormones like progesterone and thyroid hormone.

Interestingly, researchers have explored dopamine-enhancing drugs like bromocriptine and cabergoline as therapeutic tools in breast cancer. These medications lower prolactin levels by boosting dopamine’s action, and studies have shown that they can slow tumor growth, reduce tumor size, and even make some cancers more responsive to treatment. 

Other factors that cause high prolactin. 

• Dopamine deficiency: Since dopamine normally keeps prolactin in check. Anything that reduces dopamine tone (nutrient deficiencies, depression, drug side effects) can raise prolactin.
• Nutrient deficiencies: Vitamin B6, zinc, magnesium, and tyrosine deficiencies can reduce dopamine synthesis and regulation, leading to increased prolactin.
• Sleep disturbances: Prolactin follows a circadian rhythm, rising at night. Disrupted sleep can interfere with its normal regulation and lead to chronically elevated levels.

To balance prolactin: 

• Ensure that estrogen is properly balanced with progesterone.
• Manage stress.
• Optimize thyroid function. Natural desiccated thyroid is a good option for supplementation.
• Ensure adequate intake of key nutrients: vitamin B6, vitamin E, zinc, magnesium, and high-quality protein.
• Avoid polyunsaturated fats, which can disrupt hormonal balance and suppress thyroid function.
• Get bright light exposure early in the morning to support dopamine and circadian rhythm.
• Prioritize deep, consistent sleep to help regulate dopamine and hormone production.