Overview of the Hormones in Breast Milk

What are the Hormones in Breast Milk and What do They do?

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Breast milk contains more than just calories and nutrients. In fact, it's chock full of other ingredients, such as hormones. These hormones play a role in a baby's health and development. This is one of the many differences in the makeup of breast milk compared with formula.

Many of the hormones in breast milk have only recently been identified, and research is ongoing to determine what specifically these hormones and other components do for babies. It is believed that they offer significant health benefits, as research finds significant health advantages for breastfed babies.

Infant formula is, of course, a safe alternative to breast milk—and formula-fed babies also thrive. That said, formula does not contain the same makeup of hormones, antibodies, or enzymes. And, without all the necessary information, it's just not possible to try to recreate the precise hormone composition of breast milk in infant formula.

What Are Hormones?

Hormones are chemicals that are released into the blood from different parts of the body. They carry messages to organs and tissues to tell them what the body needs and what to do.

Hormones can be found in 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 Breast Milk

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

The levels of the different hormones in breast milk do not remain constant. In fact, they fluctuate quite a bit, particularly in the postpartum period and once menstruation resumes. So, as time goes on, your breast milk will have more of some hormones and less of others.

Prolactin

Prolactin is the hormone responsible for the production of breast milk. Colostrum, the first breast milk, has high amounts of prolactin. Following 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. 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 bodies than formula-fed infants do.

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 available to confirm this theory.

Epidermal Growth Factor (EGF)

The epidermal growth factor stimulates cell growth. 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.

Colostrum contains high amounts of epidermal growth factor. The levels go down quickly after childbirth. But a parent with a very early preemie (between 23 and 27 weeks) will have much higher levels of EGF in their 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.

Beta-Endorphins

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. Interestingly, there are higher levels of beta-endorphins in the breast milk of people who have a typical vaginal delivery, those who have a premature baby, and those who do not get an epidural during childbirth.

Relaxin

Relaxin is a hormone that plays a big role in 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 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. It tells the body to make more red blood cells. This hormone passes into breast milk and may help to stimulate the production of red blood cells in the newborn.

Cortisol

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. Studies show that people who are happy and have a positive breastfeeding experience 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 exactly what impact cortisol in breast milk has, but they believe that it may:

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

Leptin

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 lower body weight. 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. These hormones may also influence the baby's growth and development, but scientists are still exploring how.

A Word From Verywell

Breast milk contains a variety of hormones that impact a baby's growth, development, immune function, and other bodily systems. While the exact function of many of these hormones is still being investigated, these hormones are believed to be an important part of the nutritive value of breast milk and the health benefits it provides.

14 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.
  1. Section on Breastfeeding. Breastfeeding and the use of human milkPediatrics. 2012;129(3):e827-e841. doi:10.1542/peds.2011-3552

  2. Office on Women's Health. Making the decision to breastfeed.

  3. U.S. National Library of Medicine. Hormones.

  4. La Leche League International. Breast milk's circadian rhythms.

  5. Powe CE, Puopolo KM, Newburg DS, et al. Effects of recombinant human prolactin on breast milk compositionPediatrics. 2011;127(2):e359–e366. doi:10.1542/peds.2010-1627

  6. Vass RA, Kiss G, Bell EF, et al. Thyroxine and thyroid-stimulating hormone in own mother's milk, donor milk, and infant formulaLife (Basel). 2022;12(4):584. doi:10.3390/life12040584

  7. Thai JD, Gregory KE. Bioactive factors in human breast milk attenuate intestinal inflammation during early lifeNutrients. 2020;12(2):581. doi:10.3390/nu12020581

  8. Zozaya C, García González I, Avila-Alvarez A, et al. Incidence, treatment, and outcome trends of necrotizing enterocolitis in preterm infants: a multicenter cohort studyFront Pediatr. 2020;8:188. doi:10.3389/fped.2020.00188

  9. Dvorak B. Milk epidermal growth factor and gut protectionJ Pediatr. 2010;156(2 Suppl):S31–S35. doi:10.1016/j.jpeds.2009.11.018

  10. Shukri NHM, Wells J, Mukhtar F, Lee MHS, Fewtrell M. Study protocol: An investigation of mother-infant signalling during breastfeeding using a randomised trial to test the effectiveness of breastfeeding relaxation therapy on maternal psychological state, breast milk production and infant behaviour and growthInt Breastfeed J. 2017;12:33. doi:10.1186/s13006-017-0124-y

  11. Dehghan F, Haerian BS, Muniandy S, Yusof A, Dragoo JL, Salleh N. The effect of relaxin on the musculoskeletal systemScand J Med Sci Sports. 2014;24(4):e220-e229. doi:10.1111/sms.12149

  12. Frankshun AL, Ho TY, Reimer DC, et al. Characterization and biological activity of relaxin in porcine milk. Reproduction. 2011;141(3):373-80. doi:10.1530/REP-10-0401

  13. Yu Y, Shiou SR, Guo Y, et al. Erythropoietin protects epithelial cells from excessive autophagy and apoptosis in experimental neonatal necrotizing enterocolitisPLoS One. 2013;8(7):e69620. doi:10.1371/journal.pone.0069620

  14. Hinde K, Skibiel AL, Foster AB, Del Rosso L, Mendoza SP, Capitanio JP. Cortisol in mother's milk across lactation reflects maternal life history and predicts infant temperamentBehav Ecol. 2015;26(1):269-281. doi:10.1093/beheco/aru186

Additional Reading
  • Lawrence RA, Lawrence RM. Breastfeeding A Guide For The Medical Profession Seventh Edition. Elsevier Health Sciences.

  • Riordan J, Wambach K. Breastfeeding and Human Lactation Fourth Edition. Jones and Bartlett Learning.

By Donna Murray, RN, BSN
Donna Murray, RN, BSN has a Bachelor of Science in Nursing from Rutgers University and is a current member of Sigma Theta Tau, the Honor Society of Nursing.