Experimental and clinical data show that an appropriate hormonal milieu (primarily testosterone) plays an active role in maintaining normal sexual functioning. Recent clinical studies measuring total testosterone levels demonstrate that about 5% of men complaining of ED may have low hormonal levels, while about 18% may have low levels of free testosterone. Although testosterone is thought to facilitate erection by dilating the penile arterioles and vascular sinuses, its effect on the production of ED is still controversial. Elevated serum prolactin, a pituitary hormone, may cause almost 6% of ED cases and is usually associated with low testosterone. Hyper- or hyposecretion of thyroid hormones may cause sexual dysfunction as well.
The Massachusetts Male Aging Study (MMAS) assessed the impact of sex hormones on ED in 1,519 men, aged 40 - 70, at baseline. There was no association between total testosterone, bioavailable testosterone, and serum hormone-binding globulin (SHBG) with ED. Only increased levels of luteinizing hormone were associated with increased risk of ED, which may indicate a relationship between ED and testicular function independent of testosterone levels.
In certain cases, however, the major effect of decreased serum testosterone is reduced sex drive. Men who have everything intact but who have a decreased free testosterone level often get a sexual boost from a resupply of the hormone. Some older men with low serum testosterone and ED, however, may not respond to intramuscular testosterone injection. Nowadays, optimal replacement that normalizes serum testosterone within 24 to 72 hours is achieved with patches, gel, mucoadhesive tablets, and some oral tablets.