Male Endocrine System

& Hormones

The male endocrine system consists primarily of the testicles, pituitary gland, and to a lesser extent the adrenal glands.

Adrenal-Testicle Combined.jpg

ANDROGENS (Male Sex Hormones)

Androgens are the male sex hormones.  There are three main androgens – androstenedione, testosterone, and dihydroxytestosterone (DHT for short).  The male body makes 95% of androgens in the testicles – mainly in the form of testosterone.  The male body makes the other 5% of androgens in the adrenal glands that sit on top of the kidneys in the upper abdomen – mainly in the form of androstenedione (see image above for relative location of adrenal glands in the torso to that of the testicles in the scrotum).

Androstenedione can turn into testosterone.  We will not discuss androstenedione further since its effect really comes from its ability to turn into testosterone.  An enzyme called 5-alpha reductase can convert testosterone into dihydroxytestosternone or DHT for short.  DHT is a more potent form of testosterone that affects sexual health and body hair growth.  For those that are

chemistry buffs, you can see in the diagram below that the only difference between testosterone and DHT is a difference in the ring structure at the lower left of the molecule– a double bond in the ring structure replaced by hydrogens.  Testosterone and DHT are vital

in the development and the configuration of the male body from muscle mass, fat

distribution, growth of body hair, the development and function of the sexual organs and more. 

Some have heard that men can make estrogen and if women can make androgens.  This is true, but only to a small extent.  Both testosterone and androstenedione can be transformed into estrogens by an enzyme called aromatase.  Men have hardly any aromatase, so very little testosterone gets

converted into estrogens. Women actually make androgens in their ovaries; but since the ovaries also make a lot of aromatase, they transform nearly all of that

androgen into estrogen.  This difference in aromatase levels between men and women is why men largely produce androgens and women produce estrogens. 

As you can see from the image at the right, estrogen and testosterone have similar in structures.

Some men may produce enough aromatase to convert some of their testosterone to estrogen.  This can sometimes occur in men who take testosterone replacements.  In these cases, some men may need to take aromatase blockers in order to stop converting their androgens to estrogens. 

 

Testosterone Production

Testosterone is produced in the testicles in a multistep process that requires fats and cholesterol, as well as the mineral zinc.  Vitamin D is also an important switch to “turn on” testosterone production.

  • Testosterone production actually begins with cholesterol and fat.  Now don’t go eat a ton of cholesterol, but adequate cholesterol and fat intake is necessary in order for the body to produce testosterone.  So fat-free or cholesterol-free diets are not good for men, especially young men going through puberty.

  • Zinc deficiency has been linked to lowering levels of testosterone.  Zinc is a mineral that is essential for the testicles to produce testosterone.  It can be lost easily through sweating, as well as through ejaculation because it is stored almost exclusively in prostate fluid (which makes up the majority of semen).  So adequate daily zinc intake can be important.  There is evidence that men who are zinc deficient can take 30mg of zinc every  day and significantly increase their testosterone levels.  However, doses of zinc greater than 40mg have been linked with toxicity.  Zinc only seems to boost testosterone levels in men who are zinc deficient. [i] 

  • Men who are low in vitamin D have also been shown, not only to have lower testosterone levels, but tend to be more overweight.[ii]  It is thought that the testicles have a receptor (an on/off switch) that vitamin D binds to, which “turns testosterone production on.”  Trials have shown that men who take vitamin D can increase their testosterone production.[iii]   

  • Lack of, or inadequate secretion of, testosterone during puberty can lead to underdevelopment of the male reproductive system and sexual characteristics such as a less muscular body stature, limited hair growth, and genital underdevelopment.  Some of this has everything to do with testosterone production itself (muscle mass), but others are due to not enough testosterone to convert into DHT.

 

Roles and Effects of Testosterone

Testosterone has various effects on the male body.  Testosterone goes inside cells that have testosterone receptors and activates certain genes that promote and make us look and feel “male.”  Effects include:

  • Muscle growth (hence the reason men are more muscular than women on average) 

  • Fat distribution (men have a lower overall fat percentage compared to women, but  men get beer guts when women typically don’t)

  • Strengthens the bones

  • Deepening of the voice during puberty

  • High levels can cause acne and oily skin (thus the reason teenage boys often struggle with acne)

  • Sexualizes the brain, thus increasing libido (desire for sex) and sexual attraction (especially interest in the opposite sex)

  • Improved mood

  • Aggressive/domineering behaviors 

  • Growth of the penis and testicles during puberty

  • Growth and function of the sexual glands during puberty

  • Growth of pubic, axillary (armpit), facial, chest, and other body hair

  • Enhancing erectile function by triggering erections during sleep

  • Causing sperm development  in adolescents and adulthood

  • Energy (men with low testosterone/Low T often have fatigue)

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DHT (Dihydroxytestosterone)

As discussed above, an enzyme in the body called 5-alpha reductase transforms testosterone into another androgen called dihydroxytestosterone or DHT for short.  DHT is much more potent than testosterone and is critical for sexual development from the first few weeks of pregnancy all the way through puberty.  It is largely responsible for growth of sexual glands and production of sexual fluids.  It is also involved in body hair growth, as well as the main cause for balding in adult men.  Like testosterone, DHT targets specific tissues and some tissues are more sensitive to DHT than others.  These differences in tissue sensitivities vary from one man to the next and is one of the reasons why some men go bald and some men don’t or why some men have lots of chest hair and others don’t.  Hair growth and loss depends on whether or not the man’s skin and hair follicles are more or less sensitive to DHT.

Roles and effects of DHT

  • Proper fetal development of the male reproductive system (in fact, blocking DHT production can lead to severe birth defects and death of the fetus)

  • Normal sexual development during puberty

  • Body hair growth (facial, chest, abdominal, etc.)

  • Male pattern baldness (click HERE)

  • Growth of the prostate (can cause overgrowth of the prostate in older men called BPH, (click HERE)

  • Sexual fluid production (increases pre-ejaculate and semen production)

 

REGULATION OF THE MALE BODY

Deep within the brain is a structure called the hypothalamus.  It is located on the bottom surface of the brain near the middle of the head.  It is the regulation center for the human body.  It regulates a whole host of functions and emotions in the male body:

  • Outward emotions such as                                                                                crying, laughing

  • Body temperature

  • Sleep cycles

  • Appetite

  • Stress responses

  • Sexual drive (libido)

  • Hormone production and                                                                          regulation of the pituitary gland

Pituitary.jpg

Most hormone production is actually regulated through the pituitary gland which hangs off the base of the hypothalamus.  The pituitary is “controlled” by the hypothalamus in most instances.  The pituitary gland is split into two sections. 

The front section, called the anterior pituitary, is fed by blood vessels from the hypothalamus.  The rear section of the pituitary is called the posterior pituitary and is controlled by chemicals released by nerves from the hypothalamus. 

Pituitary Detail.jpg

The pituitary gland produces hormones that circulate through the blood stream to either effect other glands (like the testicles) or act directly on certain tissues in the body.  For men’s health we will focus on four main pituitary hormones – FSH (follicle stimulating hormone), LH (luteinizing hormone), oxytocin, and prolactin (PRL).

Pituitary Hormones.jpg

FSH and LH are referred to as gonadotropins because they act on the gonads (for men that’s the testicles).  FSH stimulates the testicles to produce sperm while LH stimulates the testicles to produce testosterone.  When testosterone levels are low, the hypothalamus “reads” the low levels in the bloodstream and secretes GNRH (gonadotropin releasing hormone) into the small blood vessels that lead to the pituitary gland.  The anterior pituitary then responds by producing FSH and LH and releasing them into the main blood supply of the body.  Once FSH and LH reach the testicles, the testicles respond by producing sperm and testosterone.  Once the testicles produce testosterone, it is released into the bloodstream where it can then effect the male body.  Once testosterone levels in the bloodstream are high enough, the hypothalamus “reads” this high level and reduces GNRH.  The reduction in GNRH by the hypothalamus leads to drops in FSH and LH being released from the pituitary.  The drop in FSH and LH from the pituitary signals

the testicles to decrease testosterone production.  We call this a negative feedback

loop.  This prevents the testicles from overproducing testosterone.  These rises and

falls in hormones actually occur in cycles.  For example, FSH surges every 90-120

minutes to promote a nearly continual production of sperm in the testicles.  Testosterone levels, on the other hand, surge upwards during sleep.  In fact, testosterone levels are highest in the early morning (one reason men often wake up with an erection or with a high libido).

Negative Feedback.jpg

Oxytocin is another pituitary hormone involved in male reproductive health.  Unlike FSH and LH, oxytocin is released by the posterior pituitary by nerve signals from the hypothalamus rather than hormones from the hypothalamus.  This allows oxytocin to be released extremely quickly.  Unlike gonadotropins which stimulate the testicles, oxytocin doesn’t stimulate another gland, it acts directly on the body tissues.  Once in the bloodstream oxytocin causes smooth muscles to contract.  Oxytocin is often thought of only in women, where it is released during childbirth to cause labor contractions in the smooth muscles of the uterus.  It is also involved in nursing, where oxytocin is released due to nipple stimulation and causes contraction of the mammary (milk) glands to secrete milk.  Oxytocin also has effects on the emotional centers of the brain where it causes a sense of “bonding” or emotional closeness.  For this reason, oxytocin is often called the bonding hormone and explains why mothers bond to their babies after childbirth and during nursing. 

 

Oxytocin plays a huge role in men’s sexual health that is often overlooked.  Sexual stimulation involves nerves.  This stimulation causes some oxytocin to be released; which helps, to some degree, with men getting an erection.  When this nerve stimulation reaches a critical point (orgasm), nerves in the hypothalamus trigger the pituitary to release a flood of oxytocin.  This flood of oxytocin causes some smooth muscles to contract. In men this occurs in the reproductive glands causing them to contract, thus assisting with ejaculation of semen.  Even the epididymis will contract to push more sperm up to the other sexual glands.  At the same time, the bonding effects of oxytocin causes the man to emotionally bond with his sexual partner.  For example, a man having intercourse with his wife would release oxytocin at orgasm

leading to ejaculation and emotional bonding to his wife, all at the same time. 

In addition, nipple stimulation can increase oxytocin release, which may explain why nearly four out of five women become sexually aroused with nipple stimulation and nearly half of men become sexually aroused or more sexually aroused.[iv]  In fact, many men report more rigid erections or a shorter time to ejaculate when their nipples are stimulated. It may have everything to do with increasing oxytocin levels.

Prolactin & Oxytocin.jpg

Prolactin is another pituitary hormone often associated with nipple stimulation.  It is most often thought of in its role in women during lactation (milk production).  However, it has a completely different role in men.  It causes a loss of libido or sexual desire and sexually shuts a man down.  Prolactin is released immediately after ejaculation and orgasm.  Its effect on the male body leads to the loss of an erection and  a decrease in libido.  This explains why men often lose interest in having sex right after ejaculating.  In addition, the same study that found that half of men become sexually aroused when their nipples are stimulated also found that about 7% had a significant decrease in sexual arousal if their nipples were stimulated during sexual activity.[iv]  This is most likely due to nipple stimulation increasing prolactin levels in these men.  In addition, high levels of prolactin lead to drowsiness, which explains why many men often fall asleep or have difficulties staying awake right after sexual intercourse. 

[i] Prasad AS, et al Zinc status and serum testosterone levels in health adults, Nutrition 1996; 12(5): 344-8.

[ii] (http://www.medscape.com/viewarticle/845483 - last viewed 9/21/16). 

[iii] Pilz S, et al, Effect of vitamin D supplementation on testosterone levels in men, Horm Metab Res 2011; 43(3): 223-5

[iv] Levin R, Meston C, Nipple/Breast Stimulation and Sexual Arousal in Young Men and Women, J Sex Med,2006;3:450-454

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