Men need testosterone to produce sperm, have sex, and father children. Your body starts to make testosterone before you're even born. By age 17, your testosterone is at its peak. It's responsible for your manly appearance, your sperm production, and your sex drive. Fertility, on the other hand, is the ability to make a baby. Infertility is not being able to conceive after at least one year of trying. About 15% of couples experience infertility. About a third of the time, the problem is with the man. Causes of Male Infertility Making healthy sperm takes place in your testicles. Testosterone helps your sperm mature. If you don’t have enough testosterone, you will produce fewer sperm than normal. However, low testosterone is not the most frequent cause of male infertility. Most of the time, the cause is enlarged veins in your scrotum, called varicoceles. These veins decrease sperm production by raising the temperature around your testicles. Urologists treat varicoceles with open or minimally invasive surgery. Other causes of male infertility include: Use of drugs and alcohol Smoking Stress Obesity Advanced age Testosterone Treatment and Infertility It’s true that your testosterone level goes down naturally as you age. Low testosterone is rare before age 40. But today, 25% of men having children are older than 35. It's no longer unusual for men to father children in their 50s. Between ages 40 to 60, you have about a 7% chance of being low in testosterone. If you've been diagnosed with low testosterone and you still want to have children, there's one thing you should know: Testosterone replacement therapy can cause infertility. When you're on testosterone treatment, your brain senses the available testosterone and reacts by telling your testicles to shut down. This can cause your testicles to shrink and your sperm production to slow down. It may take up to one year after you stop taking testosterone, for your sperm production to get back on track. This is an important point in light of a recent survey of more than 7,000 urologists: 25% have prescribed testosterone to treat infertility, even though this treatment may do just the opposite. Talk openly with your health care provider about your needs and life plans before starting testosterone replacement therapy. If you and your partner have been trying to get pregnant for more than one year, ask your health care provider if you could be dealing with male infertility. Your provider can do a semen analysis to check your sperm count and sperm health. Discuss all the possible causes and treatments of your infertility. In most cases, you can successfully treat male infertility. Key Takeaways You need testosterone for normal sperm production. Low testosterone is just one cause of male infertility; another is testosterone therapy. Talk with your health care provider if you and your partner have been unsuccessfully trying to get pregnant for more than one year. Most cases of male infertility can be treated.