You've got male: The wide-ranging effects of testosterone

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Endocrinologist and hormone researcher Prof Jeffrey Zajac talks about the broad and transformative effects of testosterone on the male body.

DYANI LEWIS Im Dyani Lewis. Thanks for joining us. Testosterone, it's the very essence of maleness. It evokes images of raw strength and competitiveness, virility and square-jawed male maturity. Testosterone is what makes a man a man, and without it we wouldn't get from one generation to the next. But testosterone and other male hormones, collectively known as androgens, have effects that reach far beyond simply setting in motion sexual maturity for half of our species. So what happens when someone lacks this body morphing hormone? Can you have too little or too much testosterone? What role does it play in the physiology of females? To tease out some of the remarkable effects of this ubiquitous hormone, I'm joined in the Up Close studio by endocrinologist Professor Jeffrey Zajac, who is the Director of Endocrinology at the Austin Hospital and Head of the University of Melbourne Department of Medicine. Welcome to Up Close Jeffrey.

JEFFREY ZAJAC Good morning. Thank you for having me.

DYANI LEWIS Jeffrey, some of the most striking effects of testosterone are in how it alters the male body during development. Could you start by telling us a bit about what sort of physical changes testosterone induces in the male body.

JEFFREY ZAJAC The effects of testosterone are time-dependent, so there are three times in life when testosterone goes up; there's a little burst during intra-uterine development, there's a short time after birth in the neo-natal period, and then the most obvious burst is at puberty in boys. So when boys go through puberty they change from pre-pubertal non sex-driven people to sex-crazed male teenagers and that effect is almost entirely due to the secretion of testosterone from their testes. So as the testosterone goes up, it makes their muscles grow, it makes their body shape change, it makes their bones grow, it changes the way the brain works and it causes all those effects we recognise in teenage boys. Then over a period of time those effects stabilise and they grow into adult males, and the male body shape, the sexual function and lots of other factors about the way males operate is determined and regulated by testosterone.

DYANI LEWIS Now puberty is obviously the period where we most noticeably recognise these changes, but what about the other peaks that happen before birth, and then immediately after, do we know what those peaks are for?

JEFFREY ZAJAC Well, yes and no. The peak during development in the uterus is required to switch on the development of the male sexual organs, the external and the internal sexual organs, so if you don't have that peak of testosterone during development the male organs don't develop and what you might call the default state develops, which is the female body shape and genital pattern. Nobody really knows what the neo-natal spike does. Grandmothers often giggle when their little grandson pees in the air with his penis erect and it turns out, not only is that person peeing, but they're already having an erection and usually if you point that out to Grandma, she gets a bit of a shock. So presumably, the main function of testosterone at that stage is not that, but it may have something to do with activating mechanisms in the brain or other tissues, but it's not really clearly understood.

DYANI LEWIS Then at each time after these peaks, does the testosterone level fall back down again?

JEFFREY ZAJAC Yes, so during most of the pre-pubertal life, the levels of testosterone are the same in males and females.

DYANI LEWIS Then even after puberty the levels fall down again as well?

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You've got male: The wide-ranging effects of testosterone