Testosterone therapy is specifically approved to treat abnormally low testosterone levels associated with symptoms and signs caused by low testosterone; a condition known as male hypogonadism. Blood tests determine whether testosterone levels are in the normal range, generally 300 to 1,000 ng/dL, but what’s “normal” may differ depending on the laboratory that conducts the test, according to The Endocrine Society’s Clinical Guidelines. Testosterone levels decline about 1% – 2% each year as men age, the guidelines report. And some studies estimate that almost 14 million men in the U.S. over the age of 45 have below normal testosterone levels. Find out if you have low t.