Prolactin

Prolactin is a Protein synthesized and secreted by Lactotrophs (acidophil cells) in the anterior pituitary
1. Stimulation (releasing factors)
  1. Vasoactive intestinal polypeptide (VIP)
  2. Thyroid Releasing Hormone (TRH, short-term factor only)
  3. Estrogen
  4. Postulated, specific Prolactin releasing Hormone (as of 2022, not identified)
2. Inhibition
  1. Hypothalamic Dopamine (Prolactin Inhibiting Factor or PIF)
Prolactin Characteristics
1. Polypeptide Hormone with 198 Amino Acids
2. Resembles Human Growth Hormone (hGH)
3. Resembles Human Placental lactogen (hPL)
Prolactin Effects
1. Prolactin stimulates milk secretion into the Breast alveoli in pregnancy and Lactation
2. Oxytocin stimulates myoepithelial cells to contract, expressing Breast Milk from the nipple
3. Prolactin response assumes prior Breast development
  1. Breast ducts, fat and stroma develop with Estrogen exposure
  2. Breast glands, alveoli and secretory tissues develop with Progesterone exposure
Prolactin increases with infant Breast Feeding
1. Increases with each Breast Feeding episode
2. Prolactin levels gradually level off after the first few months of regularly Breast Feeding
3. Prolactin decreases to normal within 1-2 months of stopping Breast Feeding

Adult: <20 ng/ml
Newborn: 100 to 300 (falls below 20 after 6 weeks)
Pregnancy
1. First Trimester: <80 ng/ml
2. Second trimester: <160 ng/ml
3. Third Trimester: <400 ng/ml
Lactation
1. Initially (<3 months postpartum)
  1. First week: 100 ng/ml basal level
  2. First 1-2 months: 50 ng/ml basal level
  3. Suckling raises Prolactin 10-20 fold above basal
2. Later (3-6 months postpartum)
  1. Basal rates approach non-pregnant levels
  2. Suckling may double basal level
References
1. Bakerman (1984) ABCs of Lab Data, p. 342