The hormone that exists in the serum may initially be bound to a protein that allows it to be transported in the body. This protein is called sex hormone binding globulin (SHBG). The total testosterone consists of two forms of testosterone; one is bound to SHBG and the other is free-circulating testosterone unattached to serum proteins.
In conditions such as hypothyroidism and cirrhosis, measurement of the total testosterone level may be normal but the patient may have symptoms of low testosterone. In these conditions, the SHBG is increased and this decreases the amount of testosterone that is available for use. The opposite is true where SHBG is decreased and the free testosterone levels are high. This situation is seen in men with obesity and hypothyroidism; these men have normal amounts of testosterone available for use by the body but the serum testosterone level is decreased. Saliva or urine test results are considered a good representation of free hormone levels.
Because SHBG is known to be decreased in cases of hypothyroidism, obesity and acromegaly, and increased in hyperthyroidism and estrogen therapy, it is necessary to measure the free biologically-active testosterone in these conditions, when total testosterone can be misleading. If testosterone is low, a full hormonal evaluation consisting of LH (luteinizing hormone), prolactin, FSH and TSH may be recommended.
Once it has been determined that the testosterone level is low, measuring the LH can help determine the cause. This separates the patients who have testicular disease from those who have diseases of the pituitary gland, the master gland that sits at the base of the brain and essentially controls many bodily functions. A low testosterone level may indicate an elevated prolactin level as well – a condition known as a hyperprolactinemia. Elevated prolactin can decrease testosterone levels by decreasing the secretion of the hormone that tells the testicle to produce testosterone.
Average levels of blood testosterone for males should range between 300-1,000ng/dl, which is too wide a spectrum when considering athletes. Athletes who need strength should have 800ng/dl or higher to optimize strength gains, hasten recovery and to have the aggressive drive to compete. Unfortunately, the average drug-free athlete is probably somewhere around 500ng/dl, which is too low.
Unfortunately, there is no general agreement on the threshold of testosterone value below which a man would be considered hypogonadal. In the United States, levels below 200 to 300 pmol/l from a morning sample are generally considered low.