Testosterone is the main androgen hormone in men. Although testosterone is present in males and females, its synthesis and secretion in women is very low compared to men.

Testosterone is a growth promoting hormone. It facilitates the growth of muscles and bones and results in increased muscle mass and strength. Testosterone is responsible for the secondary sex characteristics of males, such as voice deepening, growth of axillary hairs and beard, and maturation of sex organs. Testosterone also plays an important role in maintaining fertility in males, as it's needed for sperm development.

The normal value of total testosterone in an adult male generally lies between 270 ng/dL and 1070 ng/dL. The table below gives the normal values with increasing age.

Table: Normal Testosterone Levels

* 95% range (95.4% to be exact) refers to the range within which 95% of the measured values lie.

Normal serum testosterone levels vary with age. It is very low during childhood and then increases in puberty. Serum testosterone levels increase much more in males than in females during puberty. By early adulthood, it is around 500 ng/dL to 700 ng/dL in men, but only 30 ng/dL to 50 ng/dL in women. The serum concentration slowly declines as men age.

Furthermore, not all serum testosterone is functional or bioavailable. A serum protein called sex hormone-binding globulin (SHBG) binds nearly 40% of the testosterone in the serum. Only 2% is free and the rest (approximately 60%) is bound to another serum protein called albumin. The highest serum testosterone level occurs in the morning at around 8.00 A.M. and lowest occurs around 8.00 PM

The "Average" Value Trap

Men should be careful when reading their testosterone values as measured in the laboratory. Every lab report will provide its normal range (it may vary slightly from lab to lab) and discuss whether the values are normal or not.

However, one may be confused by the information available on the Internet, especially those selling herbal remedies or other substances to raise testosterone levels. Often they report the average value and may not mention the full range. The serum concentration of testosterone is very variable among normal people and a measured value less than the average does not necessarily mean that one needs an exogenous androgen analogue or testosterone. The table above mentions the 95% range in the third column. This range is large due to great variations in the serum testosterone levels of average people. The 95% range refers to the limits of testosterone levels in which 95% of normal people exist. It's an excellent measure of the normal range.

You can clearly see that the normal range is quite wide, and an adult may be normal with a serum concentration of 300 ng/dl or with a serum concentration of 1000 ng/dl.

High and Low levels of Testosterone

Clearly defined clinical entities associated with very high or very low levels of testosterone are often easy to recognize and diagnose. The symptoms are severe and quite characteristic, and the measured values of the serum testosterone levels are either too high or low.

High levels of testosterone could be due to tumors of the testes, androgen resistance, congenital adrenal hyperplasia, or many other conditions. The symptoms are generally marked and quite specific and include voice changes, acne, mood changes (irritability, anger, impulsivity, aggression, depression), increased muscle mass, changes in the body habitus, excessive facial hair, increased sexual desire, headaches, etc. Treatment depends on cause and might include androgen antagonists or surgical, if the cause is a tumor.

Very low levels of testosterone (often <200 ng/dL) might occur secondary to a number of conditions. Damage to testosterone producing cells in the testes may occur when the testes are affected by disease (infection, systemic illnesses, injury, tumors, radiation or chemotherapy). Afflictions of the pituitary and hypothalamus (tumors, etc.) may also result in low testosterone levels. These conditions can be diagnosed easily and treated accordingly, testosterone analogues and/or surgery (depending upon the cause). The symptoms include reduced body hair, loss of muscle mass and strength, low sexual desire and erectile dysfunction, infertility, gynecomastia, difficulty in concentrating, etc. Various conditions such as chronic illness, stress, obesity, and improper nutrition may result in low testosterone levels. The low testosterone levels in these conditions are often reversible if the cause is treated appropriately. For example, weight loss in obese individuals often results in increased testosterone levels.

Testosterone levels that are slightly lower than the normal and are associated with some vague symptoms represent a diagnostic and therapeutic challenge. In patients with total testosterone levels between 200 and 350 ng/dL, a repeat measurement along with measurement of free testosterone is recommended. Some cases with severe symptoms are being referred to as late onset hypogonadism (LOH).

Late Onset Hypogonadism (LOH) or Partial Androgen Deficiency of the Aging Male (PADAM)

LOH or PADAM is still a subject of heated debate and there is no clear consensus on its diagnostic criteria or management. The hallmark of this condition is an age-related decline in serum testosterone concentration, primarily due to testicular dysfunction. None of the causes of low testosterone mentioned above should be present. In LOH, the total serum testosterone concentrations should be less than 230-319 ng/dL and free testosterone should be less than 6.4 ng/dL. It's not known whether these patients benefit significantly from the testosterone replacement therapy.

The three most important symptoms are loss of morning erection, erectile dysfunction, and low sexual desire. Other symptoms include decreased 'energy' and mood symptoms. However, these are non-specific symptoms and are not necessarily due to low testosterone levels, even if the total testosterone is slightly reduced.

Current consensus is that testosterone replacement therapy should only be used in patients with clearly low levels of testosterone (testosterone below 250 ng/dL) in the presence of other detrimental symptoms. All patients should be carefully monitored for the beneficial effects, and the side effects associated with therapy. Testosterone replacement therapy results in improved sexual activity, increased energy, muscle mass, and bone density. However, the benefits have been proven only in patients with clear androgen deficiency.

Testosterone replacement therapy has been shown to have some benefits in aging males but mainly affects muscle, fats and bone mineral density. In an aging male, the beneficial effects of testosterone replacement therapy on sexual function, mood, muscle strength and quality of life are still topics of debate.

Testosterone analogues are dangerous drugs with immense potential for adverse effects and abuse. Routine use of testosterone replacement therapy in the absence of clear indication is not recommended, due to concerns regarding an increased incidence of prostate cancer and cardiovascular disease.