It is well-documented that testosterone levels decline with age in men.
After the age of 40 years, total testosterone decreases on average -4 ng/dL ( -0.124 nmol/L) per year  or 1.6% per year , and bioavailable testosterone by -2 to 3% per year. 
In older men (over 60 years of age), the average rate of decrement in total testosterone levels has been found to be 110 ng/dL every decade.
However, the relative contributions of changes in health and lifestyle to that decline have not been adequately evaluated. A notable study was set out to investigate this...
The objective of this study was to establish the relative importance of aging, health, and lifestyle in contributing to the testosterone decline in aging men.
The consequences of low testosterone levels (aka low-T) have been primarily investigated in middle-age and older men. However, low-T in young men aged 20-39 years can confer health risks as well...
Low total testosterone levels are associated with an adverse blood lipid profile, which includes high TG and low HDL, [1, 2] and a decline in total testosterone levels predisposes men to increased risk of cardiovascular disease (CVD) and mortality.[3-7]
"It is dangerous to be right when the government is wrong." - Voltaire
For reasons that are not readily apparent, there appears to be a conservative political movement that opposes the use of testosterone in older men. This was clearly demonstrated by the report of the Institute of Medicine, which felt that testosterone is not yet ready for prime time and that there is still a need for studies to prove its efficacy . Along the same lines, the guidelines of the Endocrine Society on testosterone use in older men seem to be ultra-cautious . But fortunately, there are also other, more liberal guidelines and recommendations [3-5].
Probably no other medical issue has been bombarded by the influx of “expert” views from all walks of life; from endocrinologists and psychiatrists to urological surgeons and gerontologists, from the lay press to the regulatory agencies and from the pharmaceutical to the entertainment industries. The dismal result of all this free-for all cacophony of opinions is a great deal of confusion, erroneous information and significant detriment to patients and physicians alike.
Let's take an in-depth look at the reasons for the negative attitudes to male testosterone replacement therapy (I will cover post-menopausal testosterone replacement in an upcoming article), and the hard scientific data that refutes it...
Recent evidence strongly suggests that testosterone deficiency is a predisposing factor for various chronic illnesses, including cardiovascular disease, diabetes and osteoporosis.[1-3]Testosterone deficiency has also been implicated as a modifiable disease risk factor for various chronic diseases in otherwise well patients.[4-7]
Cardiovascular disease, diabetes and osteoporosis-related fractures consume a significant portion of the $2.3 trillion in annual U.S. health expenditures. The economic impact of diabetes is estimated at $503 billion, $152 billion for cardiovascular disease, and $6 billion for osteoporosis-related fractures.[8-10]
Thus, the total burden of these diseases is over $660 billion, representing approximately 29% of all U.S. health care expenditures in 2008. Since testosterone deficiency is a potentially modifiable risk factor for these and other medical conditions, it may be responsible for substantial financial and quality-of-life burden on the U.S. health care system.
A study was conducted to specifically quantify the cost burden imposed by consequences of testosterone deficiency ...
Low levels of testosterone in men may contribute the development of insulin resistance and diabetes.[1-4] However, few studies have examined the association between testosterone levels and diabetes in men in the general population.
An interesting study was cunducted to test the hypothesis that low normal levels of total, free, and bio-available testosterone are associated with prevalent diabetes in men.
Hypogonadism, aka testosterone deficiency or low-T, is primarily diagnosed by low total testosterone levels. However, more and more research is showing that free testosterone, which is the active fraction of total circulating testosterone, is independently associated with important health outcomes.
Levels of free testosterone decline more steeply than total testosterone as menage.[1-7 In many cases, total testosterone levels can be relatively high, but free testosterone low. Therefore it is important to assess both total and free testosterone levels in order to get a clear picture of the androgen status.
A recent study specifically assessed if baseline testosterone (total and free) levels predict muscle loss in middle-aged and elderly Japanese men over a 10 year period.
The first step (aside from identifying symptoms) in diagnosing testosterone deficiency, aka low-T, is to do a blood test.
Here I cover some important practical things to know about a blood draw for testosterone analysis...