Overview of the Hormones in Breast Milk

What are the Hormones in Breast Milk and What do They do?
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When you're thinking about whether or not you're going to breastfeed your baby, it helps to get all the information you can about infant formula and breast milk. There are many differences in the makeup of breast milk and the makeup of formula. One of these differences is in the types and amounts of hormones found in each.

Many of the hormones in breast milk have only recently been identified, and research is ongoing as scientist continue to try to determine which other hormones and components they can find. At this point, not enough is known about these hormones. It isn't clear what many of them do for newborns and children, or why they're important. So, without having all the necessary information, it's just not possible to try to recreate the hormone composition of breast milk in infant formula.

Infant formula is, of course, a safe alternative to breast milk, but it isn't a complete source of nutrition like breast milk. With formula, there will always be something missing in the composition of nutrients, antibodies, enzymes, and even hormones.

What Are Hormones?

Hormones are chemicals that are released into your blood from different parts of your body. They carry messages to your organs and tissues to tell them what your body needs and what to do. Hormones can be found in your blood, urine, saliva, and breast milk. Hormones have many jobs. They control reproduction, growth and development, metabolism, blood pressure, and other important body functions.

The Hormones in Your Breast Milk

Your breast milk contains many hormones that pass into it from your body. Some hormones are smaller with a simple structure so they can to move more easily into your breast milk. Other hormones are larger and may not pass into the breast milk well, or at all.

The levels of the different hormones in your breast milk do not stay the same. As time goes on, your breast milk will have more of some hormones and less of others.

Here are some of the hormones that are found in breast milk.


Prolactin is the hormone responsible for the production of breast milk. Colostrum, the first breast milk, has high amounts of prolactin. But, after the first few days of breastfeeding, the amount of prolactin goes down quickly. After that, the levels of prolactin in breast milk are about the same as the levels of prolactin in the blood.

Thyroid Hormones: TSH, T3, and T4

Thyroid hormones are made by the thyroid gland. They perform many important functions, and they affect almost every system in the body. The most important function of the thyroid hormones is to control how the body breaks down food and turns it into energy. This process is called metabolism. But, thyroid hormones also regulate breathing, heart rate, digestion, and body temperature. And, they play a vital role in growth and development.

Thyroxine (T4) levels in colostrum start out low, but they go up during the first week of breastfeeding. Thyroxine may help the intestines of a newborn develop and mature. During the first few months of life, breastfed babies have much higher levels of thyroxine in their body compared to formula-fed infants.

Small amounts of triiodothyronine (T3) and thyroid-stimulating hormone (TSH) have also been identified in breast milk. It's believed that the thyroid hormones in breast milk help to protect a breastfed newborn from hypothyroidism. However, there isn't enough evidence to confirm this theory.

Epidermal Growth Factor (EGF)

The epidermal growth factor is a major growth factor that stimulates cell growth. It has many functions, but it is especially important for the development and maturation of the gastrointestinal (GI) tract or digestive system of newborns. EGF can be found in blood, saliva, amniotic fluid, and breast milk.

Right after childbirth, the colostrum contains high amounts of epidermal growth factor. The levels then go down quickly. But, if a woman has a very early preemie between 23 and 27 weeks, she will have much higher levels of EGF in her breast milk for the first month after delivery. Having more EGF in early preterm breast milk is important because babies born at this stage have a greater chance of developing GI problems such as necrotizing enterocolitis (NEC). The higher levels of EGF may help to prevent this type of serious intestinal issue.

Other growth-promoting factors including human milk growth factors I, II, and III (HMGF), and insulin-like growth factor (IGF-I) have also been identified in human breast milk.


Endorphin hormones are the body's natural painkillers. The beta-endorphins found in breast milk are believed to help newborns deal with the stress of birth and adjust to life outside of the womb. There are higher levels of beta-endorphins in the breast milk of women who have a normal vaginal delivery, a premature baby, and those who do not get an epidural during childbirth.


Relaxin is a hormone that plays a big role in female reproduction. Relaxin, as you may have guessed from the name, relaxes or loosens muscles, joints, and tendons. During childbirth, relaxin in the body works to help soften the cervix and loosen the pelvis to prepare for delivery. It may also have an effect on the growth of the milk-making tissue of the breasts.

Relaxin is present in early breast milk, and it continues to be seen in the breast milk for weeks after childbirth. The importance of relaxin in breast milk is still unknown, but its function may be related to the newborn's stomach and intestines. Since scientists do not fully understand all that relaxin does, research on this hormone continues.

Erythropoietin (EPO)

The production of red blood cells in the body is called erythropoiesis. Erythropoietin is a hormone that's made by the kidneys, and it tells the body to make more red blood cells. This hormone does pass into breast milk, and it may help to stimulate the production of red blood cells in the newborn.


Cortisol is often called the stress hormone. It's a steroid hormone that has many functions in the human body. In colostrum, cortisol is high, but the levels go down quickly and stay at lower levels as breastfeeding continues. Women who are happy and have a positive breastfeeding experience, have been shown to have less cortisol in their breast milk.

The amount of cortisol in breast milk can affect the amount of secretory immunoglobulin A (sIgA). IgA is an important antibody that protects a baby from illness and disease. Higher levels of cortisol are associated with lower levels of sIgA. So, it appears that high levels of stress and cortisol can interfere with the healthy immune-protecting properties of breast milk.

The scientific community is not sure what cortisol in breast milk actually does, but they believe that it may:

  • Help infants control the movement of fluids and salts in the digestive tract
  • Be involved in the growth of the baby's pancreas
  • Play a role in helping an infant deal with chronic stress


The hormone leptin is made by the body's fat tissue. It controls appetite, weight, and how much energy the body uses. The leptin in breast milk may help to control a baby's weight. Studies show that when breast milk contains more leptin, babies have a lower body mass index (BMI). So, leptin may help to prevent obesity in breastfed babies.

Other Hormones Found in Breast Milk

Other hormones identified in human breast milk include gonadotropin-releasing hormone (GnRH), insulin, progesterone, estrogen, androgens, gastrin, adiponectin, resistin, and ghrelin. 

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8 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading
  • Dvorak, B., Fituch, C. C., Williams, C. S., Hurst, N. M., & Schanler, R. J. (2003). Increased epidermal growth factor levels in human milk of mothers with extremely premature infants. Pediatric Research, 54(1), 15-19.
  • Dvorak, B. (2010). Milk Epidermal Growth Factor and Gut Protection. The Journal of Pediatrics, 156(2 Suppl), S31–S35. http://doi.org/10.1016/j.jpeds.2009.11.018
  • Lawrence, Ruth A., MD, Lawrence, Robert M., MD. (2015). Breastfeeding A Guide For The Medical Profession Seventh Edition. Elsevier Health Sciences.
  • Riordan, J., and Wambach, K.. (2014). Breastfeeding and Human Lactation Fourth Edition. Jones and Bartlett Learning.
  • Savino, F., Liguori, S. A., Fissore, M. F., & Oggero, R. (2009). Breast Milk Hormones and Their Protective Effect on Obesity. International Journal of Pediatric Endocrinology, 2009, 327505. http://doi.org/10.1155/2009/327505