Testosterone and Sex Drive

It is often said that the brain is the largest sex organ of the body (Schillaci, 2006). Yet, the brain is likely the last thing we think about when experiencing decreased libido or erectile dysfunction. Most often talked about when sexual problems occur, is the relationship between testosterone and sex drive. Testosterone and sex drive are often discussed when people, men in particular, experience a decrease in libido. Testosterone is a hormone produced by the testes and ovaries and in smaller amounts in the adrenal glands, which sit right on top of the kidneys. For men, testosterone is the key hormone that is involved in the sexual development of the testes and prostate. Testosterone deficiency is associated with decreased sex drive and a deficit in sexual functioning overall (Baum and Crespi, 2007). The average man produces approximately 10 times more testosterone than the average woman (Folland & William, 2007). So, is testosterone and sex drive related? Do having low or high levels of testosterone impact a person's sex drive and libido? If you are experiencing a lack of libido and sex drive, should your testosterone levels be tested? The aim of this article is to discuss the latest scientific knowledge on this issue.

Testosterone - Not Just another Hormone

First we need to understand what testosterone actually is. It is a hormone that is produced as part of the endocrine system of the body. The human body produces many hormones. Hormones influence the way our body and brain develops (Page, 2011). The part of the brain that is called the 'master gland' of the endocrine system is the pituitary gland; this is a small, pea-sized gland that sits at the base of the brain under the hypothalamus, in its own little bone casing. It is no small wonder that the pituitary is protected by a bone casing, as it secretes nine different hormones that all work together to regulate the body systems. The pituitary is part of a larger complex called the hypothalamic-pituitary-adrenal-gonadotropic axis. The feedback and interaction of the hormones produced by the pituitary and other hormone regulating glands controls such body systems as the immune system, the stress response, energy metabolism, digestion, mood, and sexuality (Page, 2011). The relationship of testosterone and sex drive to the pituitary is that the pituitary controls levels of testosterone in the body through proteins called gonadotropins. Gonadotropins are produced by specific cells of the pituitary, and they control sexual development, reproductive functioning, and normal growth. When testosterone levels are low, the pituitary releases another hormone, called luteinizing hormone (LH).

Testosterone - The 'Bigger is Better' Hormone

The function of testosterone is often misunderstood. Essentially, testosterone is a hormone that makes things bigger. Does that mean if you put it on your tomatoes, they would grow bigger? Not quite, but testosterone is the hormone that causes tissues of the body to grow larger (Folland & William, 2007). As noted, testosterone levels are ten times higher in men than in women. Folland and Williams (2007) reported that on average, men have 60% to 80% more muscle mass than women do, ostensibly due to testosterone levels being significantly higher in men than in women. Testosterone and sex drive in men may mean something different than testosterone and sex drive in women. While testosterone is the 'bigger is better' hormone, that does not mean it gives a person 'sex drive', increased penis engorgement, or even increased desire. Sex, sex drive, and sexual desire, to put it mildly, are a little more complicated than just low testosterone levels (Jockenhovel, 2004). Testosterone contributes to the sexual functioning realm, though it may indirectly affect desire levels (Simon, 2010).

Testosterone Deficiency and Sexual Activity

Sexual arousal and sexual desire are two separate phenomena. Experiencing low arousal levels is not the same thing as experiencing a decrease in sex drive, or desire (Simon, 2010). To better understand this, we can take a look at how testosterone affects the activity of sexual intercourse. Testosterone and sex drive interact through a complicated system of hormones. To reiterate, we already noted that testosterone is necessary for growth, reproduction, and sexual development. It also acts on other hormones in the body to affect sex drive, or libido (Smock, Albeck, & Stark, 1998). Testosterone, when released by the testicles (primarily) in men, the ovaries in women (primarily), and the adrenal glands in both sexes (secondarily), travels to the brain where it is converted into estradiol in the hypothalamus, through a process called aromatization (Santen, 1975; Rochira, et al., 2006). Aromatized estradiol signals the brain to produce another hormone, called vasopressin. This is where the story gets interesting. Vasopressin is synthesized in a part of the brain called the amygdala. The amygdala is responsible for many emotions we feel and perceive (Smock, Albeck, & Stark, 1998). Vasopressin travels out of the amygdala and 'talks' to other parts of the limbic system of the brain that are responsible for behaviors, other emotions, and actions. Testosterone and sex drive is thus related to the release of vasopressin. Why? Because, vasopressin has the effect of shutting down key part of electrical activity in the brain, in anticipation of sexual activity (Smock, Albeck, & Stark, 1998).

What Did You Say?

Yes, parts of our brain shut down in anticipation of sex and during sexual activity. This effect is a complicated agonistic/antagonistic interplay of the reward system of the brain. This has to do with how vasopressin acts within discrete neurons of the brain, and how it acts endogenously (Santen, 1975). What that means is that the same hormone, vasopressin, impacts testosterone and sex drive both by influencing what happens inside brain cells, and outside of the brain cells, in the 'endogenous' or synaptic gap and brain circulatory system (Smock, Albeck, & Stark, 1998). To put is simply, vasopressin creates a brain chemistry involved in the anticipatory reward of sexual gratification. In anticipation of sex, our brain is basically telling us, that sex is going to be fun. Testosterone and sex drive are linked in a feedback mechanism involving other complex systems and chemistries of the body.

The Brain, Testosterone and Sex Drive

Testosterone is necessary for sexual development. Testosterone is primarily produced in the testes, and is of significantly higher levels in men than women. Testosterone is produced when the pituitary is signaled that testosterone levels are low, through the use of an intermediate hormone, LH. LH travels to the testes and delivers the message that more testosterone is needed. Testosterone thus travels throughout the body. In the brain, testosterone is converted to estradiol, which signals the release of vasopressin. Vasopressin alters the brain's chemistry, increasing the output of the reward system of the brain, and getting the brain ready for sex. Vasopressin is excitatory in its chemical nature, and causes the release of neurotransmitters in the brain which travel to other parts of the body. One particular chemical messenger, acetylcholine (ACH), travels to the penis (Smock, Albeck, & Stark, 1998). The action of ACH in the penis causes the release of nitric oxide from the specific arteries in the penis. Nitric oxide causes arteries of the smooth muscles of the penis to vasodilate; blood rushes in these enlarged arteries as a result (Senbel, 2011). Hence, the man experiences an erection.

When Problems Occur

A loss of libido may signal low testosterone levels, though it is not testosterone exclusively that may be the cause of low sex drive or loss of sexual function. There could be other forces at work, which may include:

Aromatization Disorder (inability to convert testosterone to estradiol)
Hypogonadism (primarily in men)
Hypoactive Sexual Desire Disorder (primarily in women)
Klinefelter's Syndrome
Prostate Problems
Stress
Other environmental or physiologic factors

When there is a loss of libido or sexual function, testosterone replacement therapy may be warranted, since it can influence the ultimate amount of vasopressin or oxytocin that is released by the brain, affecting desire and function. See a clinician for testing and treatment options.

Summary - Testosterone and Sex Drive

The relationship of testosterone and sex drive can be put in the following simplified feedback loop diagram:

So, is summary, the points you need to know about testosterone and sex drive are:

The brain is the most important sex organ
The pituitary tells the testes to make testosterone
Testosterone converts to estradiol, then to vasopressin in the brain
Vasopressin causes release of acetylcholine, which releases nitric oxide into penile tissue
Nitric oxide dilates penile arteries, allowing increased blood flow, and hence erection

Arousal and Sexual Functioning: The Complicated Nature of Testosterone and Sex Drive

The reasons that we experience desire or arousal are environmental. Environmental inputs, such as the sight of an attractive partner and the smell of someone's hair or skin signal the onset of anticipation of and desire for sex. Testosterone and sex, in the man, means a huge rush of vasopressin to the brain, starting the sexual functioning process; bonding is achieved through sexual intercourse. In the women, estradiol functions differently in the brain; it causes a release of oxytocin, which for women causes arousal through bonding. The different mechanisms by which testosterone causes changes in men and women is likely housed in deep time, with human evolution demonstrating that men and women bond through sex, though by different mechanisms and for opposing yet symbiotic reasons. Problems that may occur with sex drive and sexual functioning should not immediately be attributed to testosterone levels, though diagnostic testing can rule out any issues and point the clinician and patient in the right treatment direction.