Testosterone. Most people will likely think of it as the male sex hormone, a substance vital to the health and wellbeing of men.
And while that’s mostly correct, there are plenty of popular myths about testosterone that stray far from scientific fact. From its role in developing fertility, aggressive behaviour and even strong leadership, recent research has shed new light on old beliefs and half-truths about the hormone.
To explain this and much more, we sat down with Dr Channa Jayasena, an endocrinologist based at Imperial College London.
What is testosterone?
Testosterone is a hormone, a signalling molecule. It is made mostly in the testes of men, but it can be made in smaller amounts in the ovaries of women and the adrenal glands of both men and women.
What it does is whizz around the body and act on tissues to do specific things. These include affecting behaviour, developing reproductive function, giving rise to physical changes – from growing armpit hair, facial hair – and allowing boys, particularly, to go through puberty.
Testosterone is part of a sort of superfamily of what we call steroid hormones – steroid means that they are fat soluble and are munched up by the liver. Oestrogen is another one and adrenal hormones, in general, are steroid hormones.
Are people with higher levels of testosterone more aggressive?
No, I'm afraid not. If you look at the male population, there is no relationship whatsoever, in the healthy range of testosterone levels for a man, with aggression.
Now, once you go beyond that and enter the world of anabolic steroid use, where men take enormous amounts of testosterone that are orders of magnitude bigger, that induces aggression. But that is very different to the natural effects of testosterone, and I think that's where some confusion lies.
What is the connection between testosterone and fertility?
Testosterone, when it's made in the testes, is best thought of as heat escaping from a big oven. Your body, as a whole, actually experiences very low amounts of testosterone that mainly escape into the areas around the oven.
But the oven is a furnace with very high levels of testosterone. And those high levels of testosterone are needed to bake sperm. When sperm are produced, they're like half-cooked croissants and this final process is needed for spermatogenesis.
It's complex to explain. But if you start giving someone testosterone when they don't need it – perhaps if they're taking anabolic steroids, for example – you actually switch off the oven off, and paradoxically you can create infertility.
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Do elite athletes have naturally higher levels of testosterone?
You may think if you were born with very low levels of testosterone and then took part in competitive athletics, there would be small detrimental effects on your performance. But by and large, there isn't. There isn't a linear, straightforward relationship between testosterone and physical performance.
However, there is a debate when you look at the differences between the sexes about physical performance, particularly when you deal with those who are taking masculinising HRT (hormone replacement therapy).
Within the sexes, I don't think there's an effect, but it does contribute to differences in performance between sexes.
Some people claim that men with high levels of testosterone are more successful and make better leaders and businesspeople. Is there any truth in that?
I'm afraid that's completely wrong. We do know that when men that have a problem such as having loss of testis for cancer, and so have abnormally low levels of testosterone, have their levels raised back to where everyone else in the population lies, they feel more optimistic, they feel happier and their mood rises. But if they are already within the average population levels, more does not help them further.
Can high levels of testosterone reduce your empathy?
I don't think that there is a relationship between increased levels of testosterone in men and a lack of empathy. But we know that there are clear behavioural and psychological differences between men and women.
Some of those differences are underpinned by testosterone affecting cognition in ways that we don't fully understand. So, I think reading into that, this idea may be a manifestation of sex differences that we are all aware of.
This conversation has been edited for clarity and length. The full interview, which featured in the BBC Science Focus podcast series Instant Genius, is available below.
About our expert, Dr Channa Jayasena
Channa is an endocrinologist based at the Department of Metabolism, Digestion and Reproduction at Imperial College London.
His research focuses on reproductive endocrinology and has been published in The Journal of Clinical Endocrinology and Metabolism, Current Opinion in Endocrinology, Diabetes and Obesity and The World Journal of Urology.
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