Prolactin is a hormone found in both males and females and is primarily responsible for the female's ability to produce milk. Though healthy males do not seem to make use of the hormone, the presence of low levels does not seem to have any adverse or undesirable effects on men. High levels of prolactin, however, interfere with the male's ability to produce testosterone, which can lead to a number of problems.
Both males and females normally have low levels of prolactin. In males, between 2 and 18 nanograms per milliliter is considered normal. At this concentration, the hormone does not interfere with the male sex hormones and causes no ill effects. It is believed that prolactin may be linked to immune function because certain immune cells have been shown to secrete small levels of it. It does not, however, appear that this hormone is necessary for proper immune response, so higher levels that are still within the normal range may not be indicative of greater immune health.
In mammals, prolactin is the primary hormone responsible for the production of milk and the enlargement of the mammary glands. Though only females nurse their young, males do have underdeveloped mammary glands. One of the effects of prolactin in males is that it can cause these glands to swell and, rarely, the presence of high levels of this hormone over a long period of time can cause males to produce small amounts of milk.
High levels of prolactin are also linked to low levels of testosterone. A deficiency of this male sex hormone can seriously interfere with a male's sex characteristics. The presence of too much prolactin in males can lead to the loss of sexual desire and a significantly lowered sperm count. Over time, too much can lead to temporary infertility and impotence. These symptoms go away when a male's prolactin level is returned to normal.
In addition to affecting sex characteristics, high levels of prolactin in men can lead to other problems related to the lack of testosterone. Fatigue is a common effect of low testosterone, as is a loss of muscle mass and of strength. If left untreated, high levels of prolactin can lead to loss of bone density, which greatly increases the chance of breaking or fracturing a bone, and can lead to loss of height as well.