Testosterone, one of the “sex hormones,” has both anabolic and androgenic properties. Testosterone is produced by the testes and adrenal glands in the male and the ovaries and adrenal glands in the female. It travels through the bloodstream and binds to androgen receptors, affecting almost every organ in the body, especially muscle, bone, fat cells, and reproductive organs.
Testosterone can travel through the bloodstream bound to the proteins, albumin, and sex hormone-binding globulin (SHBG), or as free testosterone, which is unattached. Only about 2-5% of testosterone is freely floating in the bloodstream.
Because free testosterone and testosterone bound to albumin can easily bind to androgen receptors, they are called bioavailable testosterone. After testosterone binds to its receptor, testosterone and the androgen receptor enter the body cell and influence how your DNA (the genetic material in each cell) directs protein production.
Total testosterone is a measurement of testosterone bound to SHBG, albumin, and free testosterone. Testosterone bound to SHBG is tightly bound and, therefore, is inactive. Testosterone bound to albumin is loosely bound and, like free testosterone, can bind to androgen receptors.
When the testes secrete testosterone, about 5% to 10% is converted to a stronger androgen, dihydrotestosterone, and affects the prostate and hair follicle. An additional 0.2% is converted by aromatase into estradiol to influence bone and the brain. The rest circulates in the bloodstream, either bound or free, affecting tissues such as muscle, until the liver metabolizes it.
Since free testosterone is bioavailable, it would follow that decreased free testosterone levels would lead to the symptoms associated with low testosterone.
When 2,641 men were studied, researchers found that men with normal total testosterone levels and decreased free testosterone levels were older and in poorer health, while men with low total testosterone and normal free testosterone were more likely younger and obese. They concluded that low free testosterone is associated with hypogonadal symptoms even if total testosterone levels were normal.
Testosterone has both androgenic and anabolic effects in the body and can affect many bodily functions.
In men, testosterone helps maintain and support:
In women, testosterone helps maintain and support:
Read More: Do Women Have Testosterone
Testosterone, like all hormones, is tightly controlled in the body. Both too high and too low testosterone levels can adversely affect your health.
Hypogonadism, or low testosterone levels in men, results from a problem producing testosterone in the testes or the pituitary gland/hypothalamus.
Besides problems in the pituitary, hypothalamus, or testes, several conditions are associated with low testosterone levels, including:
Several studies have shown that testosterone levels decrease with age. The fall is gradual, but by the eighth decade, up to one-third of men may have total testosterone levels that fall in the hypogonadal range, and 50% of men had low free testosterone levels.
Studies have shown that testosterone levels fall in men at a rate of 0.4% to 2% per year, after the age of 30. There seems to be little variability in the rate of fall across ethnic groups and geographic regions. The cause of decreasing testosterone levels with age seems to be an age-related decline in Leydig (testosterone-producing) cells in the testes, reduced enzyme activity, and a reduced response by the testes to luteinizing hormone, a hormone secreted by the pituitary gland.
The Endocrine Society recommends diagnosing testosterone deficiency only in men with symptoms consistent with a deficiency and unequivocally low blood testosterone levels. This is because the threshold testosterone level at which symptoms develop is not known.
Symptoms of low testosterone in men may include:
Having an elevated level of naturally occurring testosterone is unusual. Potential causes of high testosterone levels include tumors, using anabolic steroids, and taking testosterone supplements.
However, when testosterone levels are too high, potential side effects include:
Read More: Understanding High Testosterone Levels
Testosterone levels gradually decrease after about age 30, even though the level of SHBG increases with age. Since total testosterone comprises free and bound testosterone, if total testosterone declines with age and SHBG increases with age, then free testosterone levels decline much more than total testosterone levels.
While determining total testosterone levels is important, if you are experiencing symptoms consistent with low testosterone levels (hypogonadism), low free testosterone levels may be a better indicator of hypogonadism, especially in older men.
Testosterone levels should be measured in the morning using a reliable assay test. Testing should be repeated at least once and should include total testosterone levels and free testosterone levels. Some men, especially older men, have normal total testosterone levels, but low free testosterone.
Normal lab values may vary by laboratory. However, the standard lower limit of normal for total testosterone is 300 ng/dl (10.4 nmol/liter), and the standard lower limit for free testosterone is 50 pg/ml (0.17 nmol/liter). Your healthcare provider will probably request two to three separate measurements, ideally between 8 am and 10 am.
Further lab work may be requested based on your testosterone levels, such as luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones can help isolate whether low testosterone levels result from pituitary or testicular dysfunction.
Research into the effects of low free testosterone on other aspects of health is still ongoing. Low free testosterone levels and high LH may be associated with an increased risk for cardiovascular disease. However, low free testosterone levels were not linked to an increased risk for other diseases. Therefore, it is unclear whether low testosterone plays a causative role in cardiovascular disease or is a marker for another risk factor.
The rate of decline in blood testosterone levels is different in each man and depends on their overall health, history of chronic diseases, and medication use. To understand whether you have low testosterone levels, talk with a health professional at Invigor Medical to see whether your symptoms are consistent with low testosterone and whether you should have your total and free testosterone levels checked.
While we strive to always provide accurate, current, and safe advice in all of our articles and guides, it’s important to stress that they are no substitute for medical advice from a doctor or healthcare provider. You should always consult a practicing professional who can diagnose your specific case. The content we’ve included in this guide is merely meant to be informational and does not constitute medical advice
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