Testosterone, one of the sex hormones, is strongly linked in people’s minds with masculinity. When production of this hormone starts to fall (usually in men in their late 20s or early 30s), one side effect is lost libido or low sexual desire. While the symptoms of low testosterone (also called low-T or hypogonadism) are varied, a flagging libido is a classic sign. This can affect women as well as men, although it’s not usually as dramatic for females because sexual desire in women plays out differently than it does in men.