Testosterone and secondary sex characteristics

Hormones are natural chemicals produced by living things that help to control development and behaviour. Humans have dozens of them, but if you’re a man the most important one is simple to identify – testosterone. It’s this hormone that more or less defines masculinity, and keeping it at the right level in your body is vital for both general and sexual health. If you have too little of it, or an excessive level, it can have a range of negative effects.

The most important function of testosterone is the development of male secondary sexual characteristics. Secondary sexual characteristics cover a lot of things, but basically the term means the differences between men and women that aren’t directly related to reproduction. In humans the differences between the sexes affect almost every part of the body, and most of these differences come from the balance between the two main sex hormones. Men and women have both testosterone and estrogen, but the relative levels of each vary. In fact they vary a lot – men have about eight times as much testosterone as women. This higher level is responsible for most of the obvious things that separate the sexes, so it’s no exaggeration to say that testosterone is the real man’s hormone.

Younger boys and girls have very similar testosterone levels, and physically are a lot more similar to each other. It’s just before puberty that differences start to appear, with the first sign usually being a sudden spurt of rapid growth. This is when levels of the main sex hormones rise sharply – estrogen in girls, testosterone in boys – and secondary sexual characteristics appear.

Some of the first effects of testosterone in puberty can be quite annoying. The sebaceous glands in the skin enlarge; these glands secrete an oily substance called sebum, which protects and lubricates the skin, but in puberty their rapid growth can lead to acne.

Body hair

Body hair begins to grow

Around the same time body hair begins to grow. Before puberty most people only have hair on the crown of their head, but as testosterone levels go up it starts to appear in other areas. Pubic hair develops, then spreads down the thighs and up towards the navel. Chest, back and leg hair all start to grow. Facial hair is straggly at first but thickens towards the end of puberty.
secondary sexual characteristics

The most obvious effects of testosterone are seen in the musculoskeletal structure

The most obvious effects of testosterone are seen in the musculoskeletal structure. Through puberty it stimulates the bones to grow larger and stronger, and the pattern of growth is different than in women. The shoulders broaden and the rib cage becomes deeper and wider. More bone growth takes place on the skull, giving the face a more masculine shape – the jaw, chin, nose and the arches above the eye sockets all become more prominent. At the same time muscle mass and bulk increases. Men also tend to grow taller than women but in fact this isn’t directly linked to testosterone. Women have higher levels of estrogen and it’s breakdown products from that hormone that slow then finally stop growth.

Meanwhile, as the male body continues to grow, testosterone is having other visible effects. One of these is to change the distribution of fat. Fat deposits under the skin of the face shrink, emphasising the bone underneath and giving a less rounded appearance. The way fat is stored on the rest of the body changes, too. In women layers build up around the hips and buttocks, but in men this doesn’t happen. Instead, if there is surplus body fat, it tends to collect on the belly and around the waist. Without surplus fat the male waist is smaller than the hips but the difference isn’t as great as in a woman. The hips, in turn, are narrower than the shoulders whereas in a woman it’s usually the other way round. The overall muscle to fat ratio changes, with men having a higher percentage of muscle.

Adam’s apple

The Adam’s apple enlarges, deepening the pitch of the voice

Testosterone causes other obvious changes too. The Adam’s apple enlarges, deepening the pitch of the voice. After puberty higher testosterone levels can cause continued increases in body and facial hair, but strangely may result in less hair on the top of the head; this is the cause of male pattern baldness.

As a sex hormone testosterone is also responsible for most of the changes to the male sex organs. The most obvious of these is that the penis and testicles grow larger. Inside the scrotum, spermatogenic tissue begins to develop in the testicles. It is this tissue that produces sperm, enabling male fertility. Testosterone also plays a vital role in making sure that the sperm themselves develop normally by activating Sertoli cells.

During puberty libido increases and erections become more frequent; again these changes are caused by rising testosterone levels. They’re also often the first things to be noticed if the hormone level starts to fall – loss of sex drive is a common warning sign.

As well as physical issues testosterone plays a vital role in men’s mental health. Multiple studies have confirmed that low testosterone increases the risk of depression. This creates an unpleasant feedback loop, as depression pushes testosterone levels down further. Spatial awareness – a task men normally excel at – can be reduced by testosterone levels that are much higher or lower than average. Abnormal levels are also linked to difficulty in concentrating on a task and short-term memory problems; studies show that men’s mental health is best at average testosterone levels.

So, there are a long list of ways in which testosterone plays a part in maintaining both general and sexual health. It’s obviously not good for levels to drop much below normal but there’s some good news there. Not only does testosterone promote sexual health; a healthy sex life helps maintain testosterone levels. Scientists have found that after sex more of the hormone is produced. If levels start to drop then so will sexual urges, leading to a cycle of falling libido.

Testosterone also has other, less visible effects on the body. Men who have a higher level of it are less likely to be obese. It’s even possible that testosterone can help protect against Alzheimer’s disease; low levels have been linked to a higher risk of developing the condition.