The testosterone paradox in asthma

Asthma and an excessive response to inhaled methacholine, commonly called hyperresponsiveness, are intimately linked. Therefore, factors increasing the methacholine response are expected to contribute to asthma as well. However, this is clearly not the case for testosterone, the predominant sex hormone in males. While testosterone increases the methacholine response, it is rather protective in asthma and experimental asthma. These incompatible observations are referred herein as the ‘testosterone paradox’, and the chase is now on to unravel its underlying mechanisms.