J. Bradley Layton of the University of North Carolina at Chapel Hill in Durham, N.C., said testosterone is a key male sex hormone involved in maintaining sex drive, sperm production and bone health.
While testosterone levels tend to naturally decline as men age, lower levels of the hormone do not necessarily mean an individual has hypogonadism, a condition resulting from low testosterone.
"Over the past decade, older and middle-aged men are increasingly being tested for low testosterone levels and being prescribed testosterone medications, particularly in the United States," Layton said in a statement.
"While direct-to-consumer advertising and the availability of convenient topical gels may be driving more men to seek treatment, our study suggests that many of those who start taking testosterone may not have a clear medical indication to do so."
Layton and colleagues analyzed commercial and Medicare insurance claims from the United States and general practitioner healthcare records from Britain from 2000-11. The study identified 410,019 U.S. men and 6,858 British men who began taking testosterone during this period.
The analysis also found more than 1.1 million U.S. men and 66,000 British men had their testosterone levels tested during this time.
The study, published in the Journal of Clinical Endocrinology & Metabolism, found since 2000, the number of men beginning testosterone therapy almost quadrupled in the United States while only increasing by a third in Britain. The majority of these patients had not had their testosterone levels measured recently or only had them tested only once prior to treatment.
The Endocrine Society's Clinical Practice Guidelines on testosterone therapy in adult men recommend making a clinical diagnosis of androgen deficiency -- low testosterone -- only in men with consistent symptoms and unequivocally low testosterone levels.
"In the United States, we saw a clear trend where more and more men being tested actually had normal testosterone levels and non-specific symptoms," Layton said. "This is cause for concern as research examines potential risks associated with testosterone use."
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