Prolactin and the pituitary gland

What is the milk hormone and where is it made?

Prolactin is a hormone that has an important role in supporting milk production. Known as the “milk hormone,” it is made in the pituitary, a gland at the bottom of the brain. Prolactin levels rise during pregnancy and gradually decrease after the baby’s birth, while spiking every time the baby breastfeeds. The hormone seems to allow milk to be made but doesn’t control the amount that is made. Mothers who breastfeed exclusively tend to have higher levels of prolactin than those who give milk supplements or those who express. Prolactin also causes changes in the mother's body to support the energy needs of pregnancy and breastfeeding. The pituitary can develop problems that can affect milk production.

A) Describing prolactin

Prolactin is one of the hormones made by the pituitary, a gland found at the bottom of the brain. Prolactin is called the “milk” hormone as it has an important role in supporting the production of milk. However, prolactin is produced by both men and women and appears to have a role in regulating energy balance.

It acts by connecting with receptors on cells in various parts of the body, including (Gorvin 2015):

  • Milk-producing cells
  • Fat cells
  • Immune cells
  • Cells in the pancreas (β-cells)

B) Prolactin in pregnancy and breastfeeding

1) Prolactin levels

Prolactin levels increase during pregnancy. Once the baby is born, the levels are high and also spike every time the baby breastfeeds, stimulating the breast to make milk for the next feed.  

Baseline prolactin levels decrease as breastfeeding continues but the pituitary gland will continue to release more prolactin every time the baby breastfeeds (Uvnäs-Moberg 1990).

Mothers who breastfeed exclusively tend to have higher levels of prolactin than those who give milk supplements or those who are expressing (Hill 2009; Uvnäs-Moberg 1990). Elevated prolactin levels delay the return of the mother's periods (menses).

2) The role of prolactin

Mothers need to have enough nutrients to support pregnancy and lactation. Prolactin plays a role in this by optimizing food intake, the uptake of nutrients from the bowel, fat storage, liver activity, and insulin release and action (Lopez-Vicchi 2020).

Prolactin seems to allow milk to be made but does not control the amount of milk made. Rather, milk production is generally determined by ongoing milk removal and nipple stimulation (Cox 1996; Steube 2015).

Testing prolactin blood levels is not generally helpful in identifying the cause of a low milk supply but it can be used to evaluate pituitary function.

C) The pituitary gland

The pituitary gland is a small gland on the underside of the brain.

The pituitary gland more than doubles in size during pregnancy and reaches its largest size in the week after delivery.

The pituitary can be damaged by loosing a lot of blood or experiencing a drop in blood pressure, as can occur during labour and delivery. The gland can develop a benign (non-cancerous) tumour that produces excess amounts of prolactin. This type of tumour is called a prolactinoma.

The pituitary gland has two parts: anterior and posterior, or front and back. Both are important in regulating hormones.

1) The anterior pituitary

The anterior pituitary gland makes a number of hormones:

  • Prolactin
  • Growth hormone
  • Adrenocorticotropic hormone
  • Thyroid-stimulating hormone
  • Hormones that control periods:
    • Follicle-stimulating hormone
    • Luteinizing hormone
  • Melanocyte-stimulating hormone

2) The posterior pituitary

The posterior pituitary gland contains the ends of nerves coming from a nearby part of the brain (the hypothalamus). These nerves send hormones to the posterior pituitary which stores and then releases them. One of the hormones is oxytocin, which controls uterine contractions and the let-down of milk. The other is anti-diuretic hormone, which controls the water balance in the body.

References

Cox DB, Owens RA, Hartmann PE. Blood and milk prolactin and the rate of milk synthesis in women. Exp Physiol. 1996 Nov;81(6):1007-20
 
Gorvin CM. The prolactin receptor: Diverse and emerging roles in pathophysiology. J Clin Transl Endocrinol. 2015 May 16;2(3):85-91
 
Hill PD, Aldag JC, Demirtas H, et al. Association of serum prolactin and oxytocin with milk production in mothers of preterm and term infants. Biol Res Nurs. 2009 Apr;10(4):340-9

Lopez-Vicchi F, De Winne C, Brie B, et al. Metabolic functions of prolactin: Physiological and pathological aspects. J Neuroendocrinol. 2020 Nov;32(11):e12888

Stuebe AM, Meltzer-Brody S, Pearson B, et al. Maternal neuroendocrine serum levels in exclusively breastfeeding mothers. Breastfeeding Medicine. 2015;10(4):197-202
 
Uvnäs-Moberg K, Widström AM, Werner S, et al. Oxytocin and prolactin levels in breast-feeding women. Correlation with milk yield and duration of breast-feeding. Acta Obstet Gynecol Scand. 1990;69(4):301-6