Testosterone lab testing and initiation in the United Kingdom and the United States, 2000 to 2011

New formulations, increased marketing, and wider recognition of declining testosterone levels in older age may have contributed to wider testosterone testing and supplementation in many countries.; Our objective was to describe testosterone testing and testosterone treatment in men in the United Kingdom and United States.; This was a retrospective incident user cohort.; We evaluated commercial and Medicare insurance claims from the United States and general practitioner healthcare records from the United Kingdom for the years 2000 through 2011.; We identified 410,019 US men and 6858 UK men who initiated a testosterone formulation as well as 1,114,329 US men and 66,140 UK men with a new testosterone laboratory measurement.; Outcome measures included initiation of any injected testosterone, implanted testosterone pellets, or prescribed transdermal or oral testosterone formulation.; Testosterone testing and supplementation have increased pronouncedly in the United States. Increased testing in the United Kingdom has identified more men with low levels, yet US testing has increased among men with normal levels. Men in the United States tend to initiate at normal levels more often than in the United Kingdom, and many men initiate testosterone without recent testing. Gels have become the most common initial treatment in both countries.; Testosterone testing and use has increased over the past decade, particularly in the United States, with dramatic shifts from injections to gels. Substantial use is seen in men without recent testing and in US men with normal levels. Given widening use despite safety and efficacy questions, prescribers must consider the medical necessity of testosterone before initiation.