Hormone deficiency or hypogonadism, whether primary or secondary, has been thought to impact erectile function. Approximately a third of men in the European Male Aging Study demonstrated low testosterone, suggesting that hypogonadism is overrepresented among men with ED.11 Hormone deficiency, however, is less frequently the cause of ED than diabetes or vascular disease. Many entities with a strong relationship to ED also diminish bioavailable testosterone, including obesity, diabetes, and opioid use. Other hormones involved in testosterone metabolism or availability, like thyroid stimulating hormone and gonadotropins, also may impact erectile quality, presumably through regulating bioavailable testosterone. Understanding the relationship between testosterone and ED has been impaired by a lack of standardized measurement of this hormone and the cyclic nature of its release and consumption.
In the hepatic 17-ketosteroid pathway of testosterone metabolism, testosterone is converted in the liver by 5α-reductase and 5β-reductase into 5α-DHT and the inactive 5β-DHT, respectively. Then, 5α-DHT and 5β-DHT are converted by 3α-HSD into 3α-androstanediol and 3α-etiocholanediol, respectively. Subsequently, 3α-androstanediol and 3α-etiocholanediol are converted by 17β-HSD into androsterone and etiocholanolone, which is followed by their conjugation and excretion. 3β-Androstanediol and 3β-etiocholanediol can also be formed in this pathway when 5α-DHT and 5β-DHT are acted upon by 3β-HSD instead of 3α-HSD, respectively, and they can then be transformed into epiandrosterone and epietiocholanolone, respectively. A small portion of approximately 3% of testosterone is reversibly converted in the liver into androstenedione by 17β-HSD.
Treatment depends on the underlying cause. In general, exercise, particularly of the aerobic type, is effective for preventing ED during midlife. Exercise as a treatment is under investigation.:6, 18–19 For tobacco smokers, cessation often results in a significant improvement. Oral pharmacotherapy and vacuum erection devices are first-line treatments,:20, 24 followed by injections of drugs into the penis, as well as penile implants.:25–26 Vascular reconstructive surgeries are beneficial in certain groups.
You’ve probably heard of Viagra, but it’s not the only pill for ED. This class of drugs also includes Cialis, Levitra, Staxyn, and Stendra. All work by improving blood flow to the penis during arousal. They're generally taken 30-60 minutes before sexual activity and should not be used more than once a day. Cialis can be taken up to 36 hours before sexual activity and also comes in a lower, daily dose. Staxyn dissolves in the mouth. All require an OK from your doctor first for safety.
Overall, few patients have a compelling contraindication to testosterone treatment. The majority of men with late onset hypogonadism can be safely treated with testosterone but all will require monitoring of prostate parameters HDL cholesterol, hematocrit and psychological state. It is also wise to monitor symptoms of sleep apnea. Other specific concerns may be raised by the mode of delivery such as local side effects from transdermal testosterone.
Attention, memory, and spatial ability are key cognitive functions affected by testosterone in humans. Preliminary evidence suggests that low testosterone levels may be a risk factor for cognitive decline and possibly for dementia of the Alzheimer's type, a key argument in life extension medicine for the use of testosterone in anti-aging therapies. Much of the literature, however, suggests a curvilinear or even quadratic relationship between spatial performance and circulating testosterone, where both hypo- and hypersecretion (deficient- and excessive-secretion) of circulating androgens have negative effects on cognition.
Effective treatment for erectile dysfunction is available, and for most men will allow the return to a fulfilling sex life. The side effects of the treatment for erectile dysfunction vary depending on the treatment that is used. Some may interrupt the spontaneity of sexual activity. For example, PDE-5 inhibitors typically need to be taken one hour before sex. Side effects may include headaches, indigestion, vasodilation, diarrhoea and blue tinge to vision. Other treatments such as penile injections may cause pain at the injection site, or an erection that will not go down. Treatment options need to be carefully discussed with your doctor to determine which one is best suited to you.
Instead of the hesitation with which he had accosted the cardinal a quarter of an hour before, there might be read in the eyes of the young king that will against which a struggle might be maintained, and which might be crushed by its own impotence, but which, at least, would preserve, like a wound in the depth of the heart, the remembrance of its defeat.
Given the high risk of priapism during escalation of therapy for intracorporeal injection, it is recommended that the drugs be administered in a supervised office visit initially and that the patient be given a well-articulated plan for treatment of priapism if it occurs. Escalation guidelines for alprostadil alone vary, but a general guideline is to start at 2.5 mcg and increase by 2.5 mcg to a dose of 5 mcg and then in increments of 5 mcg to 10 mcg until an erection sufficient for penetration, not lasting more than 1 hour, is achieved. If there is no response to the initial 2.5-mcg dose, escalation dosing can be slightly more liberal.34 A European prospective trial of PGE1 alone found 91% of the 54 patients completing the 4 years of the study reported good or better tolerability and satisfaction with therapy.35
Epidemiological studies suggest that many significant clinical findings and important disease states are linked to low testosterone levels. These include osteoporosis (Campion and Maricic 2003), Alzheimer’s disease (Moffat et al 2004), frailty, obesity (Svartberg, von Muhlen, Sundsfjord et al 2004), diabetes (Barrett-Connor 1992), hypercholesterolemia (Haffner et al 1993; Van Pottelbergh et al 2003), hypertension (Phillips et al 1993), cardiac failure (Tappler and Katz 1979; Kontoleon et al 2003) and ischemic heart disease (Barrett-Connor and Khaw 1988). The extent to which testosterone deficiency is involved in the pathogenesis of these conditions, or to which testosterone supplementation could be useful in their treatment is an area of great interest with many unanswered questions.
According to British Medical Journal (BMJ), the European Male Ageing Study has provided the best estimate of the prevalence of low T—defined as a combination of sexual symptoms and measured testosterone level—finding that only 0.1 percent of men in their forties, 0.6 percent in their fifties, 3.2 percent in their sixties, and 5.1 percent of men in their seventies would meet the criteria for the diagnosis.
Findings that improvements in serum glucose, serum insulin, insulin resistance or glycemic control, in men treated with testosterone are accompanied by reduced measures of central obesity, are in line with other studies showing a specific effect of testosterone in reducing central or visceral obesity (Rebuffe-Scrive et al 1991; Marin, Holmang et al 1992). Furthermore, studies that have shown neutral effects of testosterone on glucose metabolism have not measured (Corrales et al 2004), or shown neutral effects (Lee et al 2005) (Tripathy et al 1998; Bhasin et al 2005) on central obesity. Given the known association of visceral obesity with insulin resistance, it is possible that testosterone treatment of hypogonadal men acts to improve insulin resistance and diabetes through an effect in reducing central obesity. This effect can be explained by the action of testosterone in inhibiting lipoprotein lipase and thereby reducing triglyceride uptake into adipocytes (Sorva et al 1988), an action which seems to occur preferentially in visceral fat (Marin et al 1995; Marin et al 1996). Visceral fat is thought to be more responsive to hormonal changes due to a greater concentration of androgen receptors and increased vascularity compared with subcutaneous fat (Bjorntorp 1996). Further explanation of the links between hypogonadism and obesity is offered by the hypogonadal-obesity-adipocytokine cycle hypothesis (see Figure 1). In this model, increases in body fat lead to increases in aromatase levels, in addition to insulin resistance, adverse lipid profiles and increased leptin levels. Increased action of aromatase in metabolizing testosterone to estrogen, reduces testosterone levels which induces further accumulation of visceral fat. Higher leptin levels and possibly other factors, act at the pituitary to suppress gonadotrophin release and exacerbate hypogonadism (Cohen 1999; Kapoor et al 2005). Leptin has also been shown to reduce testosterone secretion from rodent testes in vitro (Tena-Sempere et al 1999). A full review of the relationship between testosterone, insulin resistance and diabetes can be found elsewhere (Kapoor et al 2005; Jones 2007).
Interest in testosterone began when farmers of old first noticed that castrated animals were more docile than their intact peers. Ditto for castrated humans. For human males with intact gonads, testosterone increases during puberty. It deepens the voice, increases muscle growth, promotes facial and body hair, and spurs the sex drive. Testosterone also is associated with personality traits related to power and dominance.
At the present time, it is suggested that androgen replacement should take the form of natural testosterone. Some of the effects of testosterone are mediated after conversion to estrogen or dihydrotestosterone by the enzymes aromatase and 5a-reductase enzymes respectively. Other effects occur independently of the traditional action of testosterone via the classical androgen receptor- for example, its action as a vasodilator via a cell membrane action as described previously. It is therefore important that the androgen used to treat hypogonadism is amenable to the action of these metabolizing enzymes and can also mediate the non-androgen receptor actions of testosterone. Use of natural testosterone ensures this and reduces the chance of non-testosterone mediated adverse effects. There are now a number of testosterone preparations which can meet these recommendations and the main factor in deciding between them is patient choice.
Dr. Ronald Swerdloff, chief of the endocrinology division at the Harbor-UCLA Medical Center and a professor of medicine at UCLA's David Geffen School of Medicine, served on the panel of experts who developed the Endocrine Society's guidelines. He is also the principal investigator for one of the 12 sites of The Testosterone Trial in Older Men, a nationwide study funded mainly by the National Institute on Aging. The study of 800 men over age 65 with low testosterone is looking at whether men using AndroGel for one year, compared to placebo, will show improvements in walking speed, sexual activity, vitality, memory, and anemia. The study will be completed in June 2015.
In one study, 9.6% reported ‘occasional’ erectile dysfunction, 8.9% reported erectile dysfunction occurring ‘often’, and 18.6% reported erectile dysfunction occurring ‘all the time’. Of these, only 11.6% had received treatment.In another study, only 14.1% of men reported that they had received treatment, despite experiencing erectile dysfunction for longer than 12 months.
The bones and the brain are two important tissues in humans where the primary effect of testosterone is by way of aromatization to estradiol. In the bones, estradiol accelerates ossification of cartilage into bone, leading to closure of the epiphyses and conclusion of growth. In the central nervous system, testosterone is aromatized to estradiol. Estradiol rather than testosterone serves as the most important feedback signal to the hypothalamus (especially affecting LH secretion). In many mammals, prenatal or perinatal "masculinization" of the sexually dimorphic areas of the brain by estradiol derived from testosterone programs later male sexual behavior.
There have been case reports of development of prostate cancer in patients during treatment with testosterone, including one case series of twenty patients (Gaylis et al 2005). It is not known whether this reflects an increase in incidence, as prostate cancer is very common and because the monitoring for cancer in patients treated with testosterone is greater. Randomized controlled trials of testosterone treatment have found a low incidence of prostate cancer and they do not provide evidence of a link between testosterone treatment and the development of prostate cancer (Rhoden and Morgentaler 2004). More large scale clinical trials of longer durations of testosterone replacement are required to confirm that testosterone treatment does not cause prostate cancer. Overall, it is not known whether testosterone treatment of aging males with hypogonadism increases the risk of prostate cancer, but monitoring for the condition is clearly vital. This should take the form of PSA blood test and rectal examination every three months for the first year of treatment and yearly thereafter (Nieschlag et al 2005). Age adjusted PSA reference ranges should be used to identify men who require further assessment. The concept of PSA velocity is also important and refers to the rate of increase in PSA per year. Patients with abnormal rectal examination suggestive of prostate cancer, PSA above the age specific reference range or a PSA velocity greater than 0.75 ng/ml/yr should be referred to a urologist for consideration of prostate biopsy.
But if a man with sleep apnea is diagnosed with low testosterone alone, taking the supplemental hormone can worsen sleep apnea. That's why it's crucial for men with low testosterone to get a thorough workup by an endocrinologist so underlying conditions that can cause low testosterone, such as sleep apnea or pituitary-gland tumors, don't go undiagnosed, Dr. Goodman says.
Psychological factors — Psychological issues such as depression, anxiety, guilt or fear can sometimes cause sexual problems. At one time, these factors were thought to be the major cause of impotence. Doctors now know that physical factors cause impotence in most men with the problem. However, embarrassment or "performance anxiety" can make a physical problem worse.
A team led by Dr. Joel Finkelstein at Massachusetts General Hospital investigated testosterone and estradiol levels in 400 healthy men, 20 to 50 years of age. To control hormone levels, the researchers first gave the participants injections of a drug that suppressed their normal testosterone and estradiol production. The men were randomly assigned to 5 groups that received different amounts (from 0 to 10 grams) of a topical 1% testosterone gel daily for 16 weeks. Half of the participants were also given a drug to block testosterone from being converted to estradiol.
medicines called alpha-blockers such as Hytrin (terazosin HCl), Flomax (tamsulosin HCl), Cardura (doxazosin mesylate), Minipress (prazosin HCl), Uroxatral (alfuzosin HCl), Jalyn (dutasteride and tamsulosin HCl), or Rapaflo (silodosin). Alpha-blockers are sometimes prescribed for prostate problems or high blood pressure. In some patients, the use of Sildenafil with alpha-blockers can lead to a drop in blood pressure or to fainting
Testosterone replacement therapy may improve energy, mood, and bone density, increase muscle mass and weight, and heighten sexual interest in older men who may have deficient levels of testosterone. Testosterone supplementation is not recommended for men who have normal testosterone levels for their age group due to the risk of prostate enlargement and other side effects. Testosterone replacement therapy is available as a cream or gel, topical solution, skin patch, injectable form and pellet form placed under the skin.
During the second trimester, androgen level is associated with sex formation. This period affects the femininization or masculinization of the fetus and can be a better predictor of feminine or masculine behaviours such as sex typed behaviour than an adult's own levels. A mother's testosterone level during pregnancy is correlated with her daughter's sex-typical behavior as an adult, and the correlation is even stronger than with the daughter's own adult testosterone level.
Alteration of NO levels is the focus of several approaches to the treatment of ED. Inhibitors of phosphodiesterase, which primarily hydrolyze cGMP type 5, provided the basis for the development of the PDE5 inhibitors. Chen et al administered oral L-arginine and reported subjective improvement in 50 men with ED.  These supplements are readily available commercially. Reported adverse effects include nausea, diarrhea, headache, flushing, numbness, and hypotension.
All NOS subtypes produce NO, but each may play a different biologic role in various tissues. nNOS and eNOS are considered constitutive forms because they share biochemical features: They are calcium-dependent, they require calmodulin and reduced nicotinamide adenine dinucleotide phosphate for catalytic activity, and they are competitively inhibited by arginine derivatives. nNOS is involved in the regulation of neurotransmission, and eNOS is involved in the regulation of blood flow.
"I am very cautious about committing someone for life to medication," said Dr. Kathleen L. Wyne, who directs research on diabetes and metabolism at Houston's Methodist Hospital Research Institute and serves on the Sex Hormone and Reproductive Endocrinology Scientific Committee for the American Association of Clinical Endocrinologists. "That does frustrate patients because they have heard about [Low T] from TV and friends."
The mechanisms by which testosterone plays a role in erectile function are not completely understood. A study evaluating the effect of testosterone on erections in surgically castrated rabbits and control animals, in which the rabbits’ intracavernosal pressures were compared after cavernosal nerve stimulation, determined that castrated rabbits had much lower pressures after stimulation than control rabbits did.  Notably, the pressures increased when castrated rabbits received exogenous testosterone replacement.
Factors that mediate contraction in the penis include noradrenaline, endothelin-1, neuropeptide Y, prostanoids, angiotensin II, and others not yet identified. Factors that mediate relaxation include acetylcholine, nitric oxide (NO), vasoactive intestinal polypeptide, pituitary adenylyl cyclase–activating peptide, calcitonin gene–related peptide, adrenomedullin, adenosine triphosphate, and adenosine prostanoids.
Erectile dysfunction in older men. Because erections primarily involve the blood vessels, it is not surprising that the most common causes in older men are conditions that block blood flow to the penis, such as atherosclerosis or diabetes. Another vascular cause may be a faulty vein, which lets blood drain too quickly from the penis. Other physical disorders, as well as hormonal imbalances and certain operations, may also result in erectile dysfunction.
Tests such as the bulbocavernosus reflex test are used to determine if there is sufficient nerve sensation in the penis. The physician squeezes the glans (head) of the penis, which immediately causes the anus to contract if nerve function is normal. A physician measures the latency between squeeze and contraction by observing the anal sphincter or by feeling it with a gloved finger inserted past the anus.
The primary nerve fibers to the penis are from the dorsal nerve of the penis, a branch of the pudendal nerve. The cavernosal nerves are a part of the autonomic nervous system and incorporate both sympathetic and parasympathetic fibers. They travel posterolaterally along the prostate and enter the corpora cavernosa and corpus spongiosum to regulate blood flow during erection and detumescence. The dorsal somatic nerves are also branches of the pudendal nerves. They are primarily responsible for penile sensation. 
Autopsy studies have found histological prostate cancer to be very common, with one series showing a prevalence of greater than fifty percent in men over age sixty (Holund 1980). The majority of histological cancers go undetected so that the clinical incidence of the disease is much lower, but it is still the most prevalent non-skin cancer in men (Jemal et al 2003). Prostate cancer is also unusual in comparison to other adult cancers in that the majority of those with the disease will die of other causes. Treatment of prostate cancer with androgen deprivation is known to be successful and is widely practiced, indicating an important role for testosterone in modifying the behavior of prostate cancer. In view of this, testosterone treatment is absolutely contraindicated in any case of known or suspected prostate cancer. The question of whether testosterone treatment could cause new cases of prostate cancer, or more likely cause progression of undiagnosed histological prostate cancer that would otherwise have remained occult, is an important consideration when treating ageing males with testosterone.
The participants were seen every 4 weeks. Blood was taken to measure hormone levels, and questionnaires were given to assess physical function, health status, vitality, and sexual function. Body fat and muscle measurements were also taken at the beginning and end of the 16 weeks. The study was funded in part by NIH’s National Institute on Aging (NIA) and National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Results appeared in the September 12, 2013, issue of the New England Journal of Medicine.
Patients at high cardiovascular risk should not be treated for ED until their cardiac condition is stabilize. These conditions include unstable or refractory angina, myocardial infarction or cerebrovascular accident within the past 2 weeks, uncontrolled hypertension, New York Heart Association (NYHA) Functional Classification III-IV congestive heart failure, high-risk arrhythmias, hypertrophic obstructive cardiomyopathies, and moderate-to-severe valvular disease.25 This class of drugs is also contraindicated in patients who use nitroglycerin or nitrate-containing compounds.26, 27
Additionally, the physiologic processes involving erections begin at the genetic level. Certain genes become activated at critical times to produce proteins vital to sustaining this pathway. Some researchers have focused on identifying particular genes that place men at risk for ED. At present, these studies are limited to animal models, and little success has been reported to date.  Nevertheless, this research has given rise to many new treatment targets and a better understanding of the entire process.