Having low testosterone levels doesn't necessarily make you gain weight, said Scott Isaacs, MD, an endocrinologist, faculty member of Emory University School of Medicine, medical director for Atlanta Endocrine Associates, and author of Hormonal Balance: How to Lose Weight by Understanding Your Hormones and Metabolism. However, when you gain weight, your testosterone levels will decrease. “Obesity is the number one cause of low testosterone,” Dr. Isaacs said. "Taking testosterone alone rarely results in weight loss. Most patients need to change their eating habits.”

If you’re overweight, exercise can improve your testosterone levels by helping you shed pounds, says Isaacs, the author of Hormonal Balance: How to Lose Weight By Understanding Your Hormones and Metabolism.

A new study has shown that long-term testosterone replacement in men brings about impressive reductions in body weight and body fat.