To the Editor The Original Investigation by Roy et al1 recently published in JAMA Internal Medicine showed in a well-executed double-blind placebo-controlled trial the efficacy of testosterone supplementation in correcting anemia among older men with low testosterone concentrations. The authors concluded that testosterone supplementation is a feasible approach to increase hemoglobin levels of both unexplained anemia as well as anemia from known causes. The authors first defined known causes of anemia by choosing specific cut-off values and subsequently categorized the remaining group of patients as having an unexplained anemia. However, regarding 2 known causes of anemia, we question whether the authors did not underestimate its prevalence.