Prolactin is a hormone produced by the pituitary gland that regulates a number of metabolic, reproductive and immune functions. It is controlled by a number of other hormones but inhibited mainly by dopamine - if dopamine is low, then prolactin becomes high - this can have a negative effect on T levels. The release of excess prolactin has been found to decrease testosterone as it inhibits the Erections depend on testosterone, but the relationship between the two is complex and not fully understood. Some men have healthy erections despite testosterone levels well below the normal range.