DEAR DR. GOTT: My husband has been fatigued for several months. He's a little heavier than his ordinary weight and thought that was the cause, but when he went to his doctor, he was found through follow-up blood work to have a low testosterone level. What on earth is that all about? We're concerned.

DEAR READER: Testosterone is a naturally occurring hormone that stimulates the growth of male characteristics. Testosterone levels normally decrease with age; therefore, it is somewhat difficult to determine a normal level. Generally speaking, the range of total testosterone is 300 to 1,000 mg/dL, with some variation noted between laboratories. Men up to the age of 40 might have a level of 660, but by the time a man reaches 65 or so, his level could be around 525. It's when that count falls to 300 that something should be done. It is estimated that low T levels affect almost 14 million men 45 years of age and older, with less than 10 percent of them being treated.

Should a young man develop enlarged breasts, smaller testes or changes in body hair, a physician might order a testosterone level. Without outward signs, however, many younger men go undiagnosed because testing is commonly "reserved" for older individuals.

Low T levels are associated with obesity, diabetes, erectile dysfunction, low libido, irritability, fatigue, an inability to concentrate, a lack of muscle strength and hot flushes.

Hypogonadism, either primary or secondary, is a term defined as a failure of the testes to produce sufficient testosterone, sperm or both. It can be caused by congenital or acquired problems in the testes, pituitary gland or hypothalamus. Primary involves the testes. Common causes include Klinefelter syndrome, undescending testicles, mumps infection, testicular injury, normal aging, chemotherapy and several other reasons. Secondary cases are associated with problems with the pituitary gland or a portion of the brain known as the hypothalamus. This may develop from tumors, HIV/AIDS, medications such as steroids and opioids, obesity, heavy alcohol consumption and more.

Risk factors include cirrhosis, renal failure, Cushing's syndrome, sleep apnea, HIV/AIDS, sickle-cell anemia, paraplegia and disorders related to depression.

Testing involves simple blood work and should be a part of a comprehensive examination and work-up. If levels are found to be low, there are a number of possibilities for therapy. Pills remain unpopular because of their association with liver failure. Injections can cause variations in levels and involve visiting a physician periodically. Transdermal patches are applied directly to the skin to allow for steady absorption. The patches, similar to large Band-Aids, can cause skin irritation. Still other options include pellets attached to the gum and implants in the upper arm, buttock or abdomen. Both come with their unique issues. The most popular option is a gel rubbed into the skin over several sites such as the upper arms, shoulders and/or abdomen.

On the bright side, successful treatment — by whatever means preferred — should result in increased energy levels, less fatigue and increased sexual vitality. Improvement in bone density and muscle mass has been noted, too. Your husband's condition is easily treatable via whatever method he and his physician deem appropriate. You will both be glad once his replacement-hormone therapy kicks in.