Although "normal" testosterone levels are defined differently in different labs, most studies define low testosterone ("hypogonadism") in men as a total testosterone of less than 300 ng/dl.
In men younger than 40 years old, there may be significant differences between morning and evening levels. Blood testing should thus be conducted in the morning, preferably before 11 AM. Since food intake may lower testosterone levels, bloods should be obtained in the fasting state.
Due to frequent variability in testosterone levels, an abnormal test should always be repeated.
Testing should not be conducted during an acute illness, as testosterone levels may decline when you are ill.
Deficiency of testosterone is very common among adult men in the United States.
Those at particular risk include men with Type 2 Diabetes (estimated prevalence of 30-50%), obese men, and men over the age of 60 years-old.
The HIM study (Hypogonadism in Men) provides a good estimation of testosterone deficiency in otherwise healthy men.
The study found that 38.7% of men 45 years or older evaluated during well patient primary care visits had testosterone levels of less than 300 ng/ml. Most of these men did not know their levels were depressed.
Male testosterone production occurs in the testicles.
Production is controlled by lutenizing hormone (LH), a hormone made in the pituitary gland.
Low testosterone levels may result from disease in the testicles (primary hypogonadism) or disease in the hypothalamus and pituitary (secondary hypogonadism).
In primary hypogonadism, LH levels are typically elevated. In secondary hypogonadism, LH levels are undetectable or inappoppriately "normal".
Measurement of blood LH levels is thus an important test in the evaluation of this disease.
Some men have abnormalities at multiple levels.
Identification of the cause of testosterone deficiency may require further blood testing. This should be discussed with your physician.
Radiologic imaging of the pituitary (typically and MRI of the pituitary with contrast) is recommended if secondary hypogonadism is suspected.
Whatever the cause, adult onset low testosterone may be associated with fatigue, reduced interest in sex, less frequent spontaneous erections, and loss of muscle strength. Fatigue is the most common symptom.
Since the symptoms are nonspecific, diagnosis of testosterone deficiency requires a high degree of clinical suspicion. If you are concerned, discuss this with your physician.
Replacement of testosterone is available via gels, patches or injections. Treatment will often result in resolution of your symptoms with improved energy and sexual function.
Once testosterone therapy is initiated, it is important to monitor testosterone levels as up to 30% of men will require a dose adjustment.
The timing of follow-up blood testing depends on which method of therapy you select.
Since testosterone therapy may result in side effects such as increased number of red blood cells and acne, close medical follow-up is very important. You should also obtain a thorough prostate evaluation by your physician before beginning therapy since Prostate Cancer is a contraindication to treatment with testosterone.