Hormones and Lactation
bonding, bone loss, rage, and more
Oct 21, 2025
Hey! Welcome back - my intention was to post this on Monday (yesterday), but it was a reset day in my house. So, instead I am coming to you with a pistachio latte and a chocolate croissant at a cute cafe. We’re continuing the hormone discussion from last week. If you missed the release of my newsletter and Nip Tips, you can check it out here:
To recap:
Progesterone is THE pregnancy hormone - it maintains pregnancy and prevents prolactin from being released.
Prolactin is responsible for making breast milk, levels rise after birth when the placenta is delivered and remain elevated as long as breastfeeding continues.
Oxytocin is the hormone of love, it is responsible for the milk “letdown” or release from the breast and plays an important role in bonding with loved ones (especially babies) and helps the uterus contract in order to control bleeding postpartum.
Prolactin
Let’s revisit prolactin - our it girl. She is the star of breastfeeding, but prolactin is present in all of us (lactating or not). It has numerous functions, but is an important hormone for reproductive health in both men and women. Before and during pregnancy prolactin is responsible for breast development, increasing size and preparing for milk production. Elevated levels outside of pregnancy and lactation can cause infertility issues and galactorrhea (milky discharge from the nipples outside of lactation) in both men and women.
Interestingly - prolactin has been attributed to animals intense protective behaviors of their babies. It has been termed the “rage hormone”. Aggressive behavior is rare in virgin mice, but maternal aggression is seen in lactating mice. Assuming this is an adaptation to defend their young from perceived threats. The studies in humans have had mixed results, as not all women with elevated prolactin levels - and certainly not ALL breastfeeding mothers experience rage. But this is an interesting area of continued study that may lead to help with postpartum mood disorders.
Elevated prolactin also decreases bone density - everyone said the baby weight would fall off, but no one told me it would be from my bones.
Breastfeeding is so reliant on prolactin that if breastfeeding does not occur postpartum, levels will return to pre-pregnancy levels by 7 days postpartum (Wambach & Spencer, 2026). Nipple stimulation is the single most important thing to raise prolactin levels and milk production. When in doubt, more nipple stimulation - breastfeeding, pumping, hand expression.
Oxytocin
Love, pleasure, bonding - oxytocin is the one. Released in response to suckling, oxytocin causes the milk ejection reflex or “letdown”. Prolactin creates the milk, but there’s no where for it go without oxytocin (Wambach & Spencer, 2026). A majority of women can feel their letdown - a warm/tingling sensation. However, there are multiple letdowns during a nursing session and most women do not feel them.
Other actions of oxytocin include increased thirst (hello - emotional support water bottle), flushing similar to hot flashes, and contracting the uterus postpartum to control bleeding.
Once the uterus loses the “baby weight” (ha!), oxytocin continues to cause pulsing contractions of the uterus that may be pleasurable to the mother (Wambach & Spencer, 2026). If this was you - stop everything and message me IMMEDIATELY! I must know who experienced this incredible phenomena. Twas not me.
Women under stress or who have emergency cesarean or c-section births have lower levels of oxytocin. Breast massage can increase oxytocin, so in women who experience traumatic births, I would encourage them to use their hands to massage their breasts while nursing to help. Women with higher levels of oxytocin experience less pain and report less stress, more calm.
Cortisol
Cortisol is so hot right now - from drinks to help lower to being blamed for puffiness in your face, this hormone is taking a lot of heat. So, we’re taking it to the adrenal glands (located on your kidneys) as opposed to oxytocin and prolactin which are coming from your brain. To be clear cortisol has important roles in stress response, metabolism, and inflammation outside of lactation, but we’re keeping it simple today and will focus on breastfeeding.
Cortisol and prolactin work together and enhance their expression, aka they’re bestie. Or breasties. Cortisol helps prolactin prepare the breast for lactation. During lactation, levels are lowered and appear to help reduce stress. If cortisol levels become too high related to stress from illness, trauma, cesarean, etc. - lactation can be delayed or milk production reduced (Wambach & Spencer, 2026).
The relationship of these hormones is a delicate balance and the relationship - just like human relationships - goes both ways! And too much of a good thing, is not in fact, a good thing.
Thyroid Stimulating Hormone (TSH)
The thyroid joins the party. TSH is the hormone that stimulates the thyroid to produce thyroid hormones (T3, T4). TSH works with all her friends discussed above to grow the breasts and prepare for lactation, there is a significant increase in TSH levels 3-5 days after birth (Wambach & Spencer, 2026). Kellymom has great information on the effects of hyper/hypothyroid disease on lactation. Women with hypothyroid or elevated TSH levels may experience low milk supply. Women with low milk supply that cannot otherwise be explained, may benefit from a thyroid check and speaking to their medical provider. Especially if they have other symptoms of hypothyroid like depression, fatigue, thinning hair, cold intolerance, poor appetite, and weight gain.
Prolactin Inhibiting Factor (PIF)
PIF/dopamine - we love dopamine, but prolactin does not. They’re both trying to be the star, but when it comes to breastfeeding we gotta support Prolactin. Sorry PIF. Nipple stimulation increases prolactin and suppresses dopamine/PIF. We should keep PIF in mind though, because understanding their relationship can help us support women who choose to stop breastfeeding for a number of reasons or to help treat people with elevated prolactin levels outside of lactation (Wambach & Spencer, 2026).
Woo - the caffeine is hitting and it’s time to wrap up this lesson in lactation. This post was science heavy for my pearlies (people whom love science), but I hope you found value in it. I appreciate your support in creating and spreading breastfeeding education to those who need it.
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*any errors in spelling, syntax, or otherwise is proof that none of this content was created by AI.
(1) Wambach, K., & Spencer, B. (Eds.). (2026). Breastfeeding and human lactation (seventh edition). Jones and Bartlett Learning.
