Looking purely at the biochemical numbers, The Endocrine Society* considers low testosterone to be a total testosterone level of less than 300 ng/dl, and I think that’s a reasonable guide. But no one quite agrees on a number. It’s not like diabetes, where if your fasting glucose is above a certain level, they’ll say, “Okay, you’ve got it.” With testosterone, that break point is not quite as clear.
^ Jump up to: a b Travison TG, Vesper HW, Orwoll E, Wu F, Kaufman JM, Wang Y, Lapauw B, Fiers T, Matsumoto AM, Bhasin S (April 2017). "Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe". The Journal of Clinical Endocrinology and Metabolism. 102 (4): 1161–1173. doi:10.1210/jc.2016-2935. PMC 5460736. PMID 28324103.
Testosterone strengthens bones. You may have thought of osteoporosis as a health problem that only women have to worry about, but men can suffer from this bone-weakening disease too. And low testosterone levels may be to blame. Testosterone has been shown to play an important role in bone health. It increases bone density by stimulating bone mineralization as well as decreases bone resorption. Elderly men suffering from osteoporosis typically have sub-optimal testosterone levels. If you want to enjoy strong, healthy bones well into old age, take steps to improve your testosterone levels now.