You may know your total testosterone levels, but they don’t tell the whole story. Your total testosterone can be divided into three categories:
Tightly bound testosterone. About two-thirds of the testosterone in your blood is bound to sex hormone binding globulin (SHBG). Your body can’t use it.
Loosely bound testosterone. About one-third of the testosterone in your blood is bound to albumin. Your body can use it, but with some effort.
Free testosterone. A small percentage of the testosterone in your blood (1–4%, as a rule) just floats around freely. Your body can readily use it, and the enzyme 5-alpha-reductase can convert it to dihydrotestosterone (DHT), a very potent androgen.
Together, your loosely-bound and free testosterone compose your bioavailable testosterone, which has a greater impact on your health than your total testosterone.
Testosterone boosters are supplements that increase your production of testosterone. Supplements that increase only your percentage of free testosterone or DHT are often included in this category.
Supplements that inhibit CYP19A1, the aromatase enzyme, are indirect testosterone boosters in men. CYP19A1 serves many purposes, one of which is to convert testosterone to estradiol, the predominant form of estrogen. Inhibiting this enzyme reduces the percentage of testosterone that gets converted to estradiol.
Contrary to what you might think, the male body needs estradiol, though in lesser quantity than women need. When the body detects that estradiol levels are too low, it reacts by increasing its production of the base material it needs to make estradiol: testosterone.
Aromatase inhibitors can boost testosterone on their own, but they can also complement other testosterone boosters. If you take a supplement that increases testosterone without inhibiting the aromatase enzyme (through hypothalamic stimulation, for instance), you may find yourself with more estradiol than you need, a situation that taking an aromatase inhibitor may remedy — if you’re a man, that is. As we saw, aromatase inhibitors hinder the conversion of androgens to estrogens; in premenopausal women, however, ovaries produce most of the estrogen, so aromatase inhibitors are much less effective.
For a list of supplements to take to boost testosterone, see our increasing testosterone page.