There is much confusion, conjecture and bro’lore surrounding prolactin and progesterone and how to control them to avoid gynecomastia, loss of libido and shutdown. A lot of confusion surrounds the misconception that these two hormones are one in the same. They are, in fact, two totally different hormones, with two totally different mechanisms of action and totally different effect profiles in the body.
Prolactin is a peptide hormone of 199 amino acids that is similar in structure to growth hormone. Prolactin is produced in the anterior pituitary in cells called lactotropes. Prolactin is secreted in an episodic fashion throughout the day. Its secretion is inhibited by dopamine and stimulated by estrogen, stress, TRH, and other factors such as suckling and nipple manipulation. Prolactin acts through prolactin receptors present on the surface of cells. In the human, these receptors are stimulated by GH and prolactin with equal potency. Prolactin initiates and maintains lactation in the estrogen primed breast. Prolactin is not a growth factor in breast tissue which is why it is necessary for breast tissue to be primed by the growth promoting action of estrogen in order for prolactin to exert its effects. Even so, lactation is prevented in the presence of high levels of estrogen and progesterone, such as those that exist in pregnancy, and lactation only proceeds with a drop in estrogen/progesterone levels post delivery. Prolactin inhibits gonadotropin secretion and therefore suppresses the hypothalamic pituitary gonadal axis and the production of testosterone.
Progesterone is a steroid hormone that binds to intracellular progesterone receptors that act in the nucleus of cells. Progesterone is produced in males by the adrenal glands and males have the same plasma level of progesterone as women d o during the follicular phase of the menstrual cycle. There are actually two progesterone receptors. A functional receptor and a nonfunctional receptor that acts to suppress the activity of the functional receptor. Progesterone antagonizes the effect of estrogen by reducing estrogen receptor levels. This is exemplified by the use of progestins to fight estrogen responsive breast cancer. Progesterone has a potent suppressive effect on gonadotropin secretion and has been used as a contraceptive agent in men.
There is no evidence that controlling prolactin will prevent or treat gynecomastia. Many of the issues that are being attributed to prolactin can be explained through other mechanisms. While reducing prolactin may help with reduced libido and shutdown due to AAS, the potential risks involved with using dopaminergics to reduce prolactin levels probably outweighs the benefits. If estrogen is controlled during a cycle, then prolactin is unlikely to be elevated and unlikely to cause a problem.