I have recently seen an increasing number of men who have developed significant degrees of clinical hypogonadism - low sex drive, erectile dysfunction, reduced sexual sensations and listlessness, fatigue and/or "brain fog" - while either taking finasteride or after stopping the medication, even long after stopping it. Finasteride is a medication approved by the FDA to treat benign prostate enlargement and testosterone-related hair loss. It does so by blocking the function of an enzyme called 5-alpha reductase. This enzyme normally converts testosterone to dihydrotestosterone (DHT), which is a more potent hormone that acts at the prostate and the hair follicles. The occurrence of these symptoms in the setting of being on, or even after stopping, the medicine presents a compelling neuroendocrine problem. And it is reasonable to assume that not all men suffer from the exact same cause.
The investigation of this problem begins with measuring blood levels of the testicular hormone testosterone, DHT and luteinizing hormone (LH) and follicle stimulating hormone (FSH), which are the pituitary gland's hormones that stimulate the testicles to make testosterone, and also estradiol, a "female" hormone that is present in men, but in much smaller amounts than in women. Small elevations in estradiol in men can have noticeable anti-testosterone effects.
If testosterone is low and LH and FSH are high, then the problem is testicular. If testosterone is low and LH and FSH are low or even normal, then the problem is "upstairs" either in the pituitary gland or in higher brain centers that control hormones. Depending on the outcome of this evaluation, treatment first and foremost involves giving enough testosterone to bring high normal levels (by gels or intramuscular shots), while blocking the conversion of some of the testosterone to estrogen, which happens naturally in fat tissue. There is a pill for this.
If high normal levels of testosterone, combined with low estrogen levels, does not relieve the hypogonadal symptoms, then the possibility that the man has some resistance to testosterone must be considered. This would be akin to the insulin resistance seen in diabetes and would be treated in a different manner. An MRI of the pituitary gland would be indicated if LH and FSH are low or normal, at the same time that testosterone is low, to exclude the possibility of a benign tumor impairing the pituitary's ability to respond to the low testosterone.
Finally, other brain hormones can be effected by finasteride. These are called neurosteroids and their decline can bring anxiety and depression. These neurosteroids cannot yet be measured commercially with blood tests, but the "brain fog" some men experience on finasteride may result from disturbances in attention caused by anxiety or mood changes.
Finasteride certainly helps men fight hair loss and prostate enlargement. However, a considerable number of men have intolerable and sometimes persistent side effects from the medicine. A systematic neuroendocrine approach to this problem should shed light on the cause in a majority of cases and bring relief.