If male pattern baldness is caused by testosterone, why are hairs on top of the head more affected by it and hairs like eyebrows, beard, inside ears etc, are affected less?
In: Biology
Everyone learns in school that testosterone is the “male” hormone, and estrogen is the “female” one. Trouble is, while that’s not wrong, it’s also not completely accurate.
It’s true that testosterone is the main precursor hormone that men produce. But many of the stereotypical effects of testosterone (body hair, hair loss, libido, aggression) are actually controlled more by the hormones your body makes FROM testosterone.
Sex drive, for example, is actually significantly driven by estrogen! Your body uses an enzyme called aromatase to convert a certain amount of testosterone to estrogen. This is why bodybuilders who inject synthetic testosterone far above their natural levels end up growing breasts, unless they actively take anti-estrogen drugs. In most guys the ratio is 20-30 to 1, but if that ratio is too high OR too low, then your sex drive, erections, etc. will not function correctly.
Another hormone your body generates from testosterone is dihydrogentestosterone (DHT), which it creates with an enzyme called 5-alpha reductase. DHT is actually responsible for both body/facial hair growth, and male pattern baldness. (It also has a very high influence on sex drive.)
Now it starts to get a little more complicated. The various parts of your body have different genetic sensitivities to these hormones. Every organ in your body has hormone receptors, basically sites that “listen” for the hormone in your blood. Some people have more or less listeners on various parts of their body than others. People with less listeners need more of the hormone to trigger a reaction, and people with more listeners need less.
DHT is primarily responsible for causing facial and body hair, but DHT is actually toxic to the hair follicles on the top of your head. The follicles are just built differently. The effect is cumulative, meaning it builds up over the years and starts eventually permanently killing the head follicles. How long until you start losing it? Well, that depends on how much DHT you produce, how sensitive your follicles are to it, and how long it’s been building up. That’s why men tend to lose it later in life. It’s also responsible for the cliche of men growing hair in unexpected places as they get older.
Women don’t lose their hair because they don’t make enough testosterone to start any of this process to begin with. But some guys with very high testosterone still never lose their hair, either because they don’t generate much DHT from it, or their hair follicles don’t have many listeners and are not genetically as sensitive to it. Some guys with relatively low testosterone do end up losing their hair, either because they produce a lot of DHT, or their head follicles have a lot of listeners and are very sensitive to it.
So now, you can see how it’s actually much more complicated than what you were taught in school! Yes, you first need to produce enough testosterone, period. But then your genetics determine how much of that is converted to estrogen or DHT. And then your genetics also determines which parts of your body are the most sensitive to them. These separately controlled genetic combinations are why men are so varied, and why a simple test of testosterone levels in your blood doesn’t actually tell you very much.
But it’s also why associating certain “mainly” traits with “high testosterone” is actually not completely accurate. It’s just complicated to explain all the rest, so schools tend to teach the dumbed-down version instead.
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