Harvey B. Simon, M.D. is an Associate Professor of Medicine at Harvard Medical School and a member of the Health Sciences Technology Faculty at Massachusetts Institute of Technology. He is the founding editor of Harvard Men's Health Watch (www.health.harvard.edu) and the author of six consumer health books, including The Harvard Medical School Guide to Men's Health (Simon and Schuster, 2002) and The No Sweat Exercise Plan. Lose Weight, Get Healthy and Live Longer (McGraw-Hill, 2006). Dr. Simon practices at the Massachusetts General Hospital; he received the London Prize for Excellence in Teaching from Harvard and MIT.
What would cause a lower than normal testosterone level?
Before you ask why your testosterone is low, you need to know if it really is low. It's a simple question with a complex answer.
Instead of a single level for testosterone, normal men exhibit a wide range, with testosterone levels between 270 and 1,070 ng/dL (nanograms per deciliter). But, like so many biological functions, testosterone production goes up and down over a 24-hour cycle. Production is highest at 8 a.m. and lowest at 9 p.m.
For measurements to be meaningful, they should be obtained at a standard time, usually first thing in the morning. There's another complexity: Testosterone travels in the blood in one of two forms, bound to one of two proteins or unbound. Only the free and the albumin-bound forms are biologically active. They are known as bioavailable testosterone. If you really need to know where you stand, you need both your total and free (or bioavailable) testosterone levels measured, preferably early in the morning.
If your levels are low, what might be responsible? Testosterone is produced by the Leydig cells of the testicles. To make the male hormone, these cells must first be stimulated by a hormone called LH, which is made by the pituitary gland in the brain. LH production depends on stimulation by the hormone LHRH, which is made in another part of the brain, the hypothalamus. So testosterone deficiency (hypogonadism) can result from problems involving the testicles or the brain.
Testicular problems may be traced to genetic errors, mumps, severe trauma, alcoholism, and chemotherapy and radiation treatments for cancer. Problems that originate in the pituitary gland include tumors (almost all noncancerous), head trauma, brain surgery, various medications, some hereditary disorders, severe malnutrition, and chronic illnesses.