One month of treatment produced a 74-second gain in exercise tolerance without changing HDL or LDL cholesterol levels. Compared with the placebo-treated subjects, men on testosterone displayed improved exercise tolerance at the end of the trial — but the difference was slight, amounting to an average gain of just 26 seconds. Only a few small, short-term studies have been published to date, and the results offer mixed support for this theory. At present, the hormone does not appear linked to hypertension or inflammatory markers. That's one of the things that gave testosterone its bad reputation. For instance, fluctuation in testosterone levels when a child is in distress has been found to be indicative of fathering styles. While the extent of paternal care varies between cultures, higher investment in direct child care has been seen to be correlated with lower average testosterone levels as well as temporary fluctuations. Fatherhood decreases testosterone levels in men, suggesting that the emotions and behaviour tied to paternal care decrease testosterone levels. Physical presence may be required for women who are in relationships for the testosterone–partner interaction, where same-city partnered women have lower testosterone levels than long-distance partnered women. Testosterone levels do not rely on physical presence of a partner; testosterone levels of men engaging in same-city and long-distance relationships are similar. Low bioavailable testosterone but not total testosterone significantly influenced the all-cause and cardiovascular mortality after the multivariate analysis (Figures 1 and 2), suggesting that this is the more sensitive assay in detecting pathological deficiency and risk compared with other assays. In these models, testosterone appeared to act directly on the vascular smooth muscle, having an antagonistic action on calcium channels similar in effect to that of the anti-anginal drug nifedipine.60In vitro studies of isolated male arteries have demonstrated similar vasodilatory actions. Further work will undoubtedly expand our knowledge regarding the underlying mechanisms and relationships between low testosterone and coronary disease and this will be of great interest. Studies in male animals have shown accelerated atherosclerosis after castration—an effect that is abrogated by androgen replacement therapy.43, 44 One question which remains unanswered is whether low testosterone levels accelerate the development of CAD or whether they are simply a consequence of chronic illness? Some studies measured total testosterone level, others used free or bioavailable testosterone or used calculated free-androgen index. Blood vessels and heart muscle cells have receptors that latch on to testosterone. There is less information about the relationship between testosterone and other cardiac risk factors. In high doses, androgens tend to raise LDL ("bad") cholesterol levels and lower HDL cholesterol levels. Many men with ED blame their sagging performance on sagging hormone levels. But the fact that large amounts of testosterone harm the heart and metabolism doesn't necessarily mean that physiological amounts are also harmful. Animals that are given testosterone develop enlarged hearts. The same study demonstrated a prevalence of hypogonadism of 24% in men with coronary artery disease, by strict criteria, which is approximately three times higher than the expected background rate. Bioavailable testosterone assays have been utilized in several studies of men with coronary artery disease and more consistently demonstrate decline with age. Multiple cross-sectional studies have demonstrated a fall in androgen levels with advancing age.16, 17, 18, 19, 20, 21, 22, 23, 24, 25 However, unlike women, men do not experience the well characterized, sudden and rapid decline in sex hormone levels and cessation of reproductive capability as they age. 5α-DHT binds to the same androgen receptor even more strongly than testosterone, so that its androgenic potency is about 5 times that of T. Free testosterone (T) is transported into the cytoplasm of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5α-dihydrotestosterone (5α-DHT) by the cytoplasmic enzyme 5α-reductase. Androgens such as testosterone have also been found to bind to and activate membrane androgen receptors. The relationship between sex steroids and SHBG in physiological and pathological conditions is complex, as various factors may influence the levels of plasma SHBG, affecting bioavailability of testosterone. Only the free amount of testosterone can bind to an androgenic receptor, which means it has biological activity. An increased risk of premature cardiovascular events in men initially led to the belief that testosterone had detrimental effects on cardiovascular health. In one recent interventional study of frail hypogonadal men, supraphysiological dosages of testosterone replacement therapy was used in an attempt to improve muscle strength.93 The study showed that pharmacological doses of testosterone significantly increased muscle strength, but the trial was stopped early because of an excess of cardiovascular side effects. In men with angiographically-proven coronary disease, 12 weeks of trans-dermal testosterone therapy significantly increased time to ischaemia at exercise testing. Testosterone may prove to be an effective treatment in female sexual arousal disorders, and is available as a dermal patch. In addition, a continuous increase in vaginal sexual arousal may result in higher genital sensations and sexual appetitive behaviors. There is a time lag effect when testosterone is administered, on genital arousal in women. Women's level of testosterone is higher when measured pre-intercourse vs. pre-cuddling, as well as post-intercourse vs. post-cuddling. Androgens may modulate the physiology of vaginal tissue and contribute to female genital sexual arousal. This reaction engages penile reflexes (such as erection and ejaculation) that aid in sperm competition when more than one male is present in mating encounters, allowing for more production of successful sperm and a higher chance of reproduction. Therefore, these mammals may provide a model for studying clinical populations among humans with sexual arousal deficits such as hypoactive sexual desire disorder.
