What's the best way to test for low testosterone?
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Q. I'm worried that I might have very low testosterone levels. How do I get tested and are there certain steps that can ensure an accurate reading?
A. Doctors usually order a blood test to measure total testosterone. A total testosterone lab test measures both "free" (unbound) testosterone and the amount of hormone bound to blood proteins. Normal testosterone levels are between 300 to 1,000 nanograms per deciliter (ng/dL).
While it's the most widely used test, caution is needed when interpreting the results. First, a lot of the testosterone bound to proteins is not active in the body. Also, the percentage that binds to proteins differs; it differs from one man to the next. Second, total testosterone fluctuates during the day. Laboratories set normal values based on men having their blood drawn between 7 a.m. and 10 a.m.
Finally, slightly low readings in middle-aged and older men don't consistently correlate with symptoms. And the measured total testosterone levels can vary from test to test and from lab to lab. For people with borderline-low testosterone levels, a repeat test would show a normal range about 30% of the time.
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About the Author
Howard E. LeWine, MD, Chief Medical Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing
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