Low Testosterone
(Low T)

Low Testosterone Summary
Low testosterone can affect both men and women. Causes of low testosterone in males include undescended testicles and injury to the scrotum. Low testosterone in females includes ovary conditions. Treatment for low testosterone in men includes testosterone replacement therapy. Currently there is no FDA approved testosterone treatment for women.
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Low testosterone facts

  • Testosterone is produced by the testes in males and ovaries in females. The testes are under the hormonal control of the hypothalamus and pituitary in the brain and make testosterone in response to stimulation by FSH and LH.
  • Primary hypogonadism occurs because of the inability of the testes to produce testosterone.
  • Causes of secondary and tertiary hypogonadism are due to a variety of illnesses or diseases that affect the hypothalamus-pituitary-gonadal axis.
  • Symptoms of low testosterone depend upon when in the life cycle it occurs.
  • Osteoporosis and loss of muscle mass are significant complications of low testosterone.
  • In the United States, testosterone replacement therapy is available as an FDA-approved treatment in men only. It may be administered by injection, patch or gel, or gum and cheek putty.

What is testosterone?

Testosterone is an anabolic-androgenic steroid hormone which is made in the testes in males and in the ovaries in women (a minimal amount is also made in the adrenal glands). Testosterone has two major functions in the human body.

  1. Testosterone is needed to form and maintain the male sex organs and promote secondary male sex characteristics (in both men and women) such as voice deepening and hair growth patterns. This function is related to its androgenic properties.
  2. Testosterone is the facilitation of muscle growth as well as bone development and maintenance. This is a result of its anabolic properties.

Testosterone production is regulated by hormones released from the brain. The hypothalamus and pituitary gland located in the brain produce hormonal signals that ultimately result in the production of testosterone. The hypothalamus is located just above the brain stem, and among its many functions, it produces gonadotrophin releasing hormone (GRH). GRH travels a short distance to the pituitary gland located in the base of the brain stimulating the gland to release FSH (follicle stimulating hormone) and LH (luteinizing hormone). These hormones travel through the bloodstream to activate the sex organs in both men and women. Subsequently, these hormones also have a role in regulating testosterone levels in the bloodstream.

The majority of testosterone circulates in the blood bound to a carrier protein (a hormone is produced in one are of the body and has its effect on another area. Often a carrier protein that assists the hormone travel through the bloodstream). In this case the carrier protein is called "sex hormone binding globulin," or SHBG. When testosterone is being carried by SHBG, it is considered "bound". Bound testosterone does not play an active role in the body; only the unbound or "free" testosterone is able to enter the different cells of the body and exert its androgenic and anabolic effects. Thus, anything that affects the function or the amount of SHBG can also affect the total circulating amount of active testosterone.

Picture of the endocrine system including the testes and ovaries
Picture of the endocrine system including the testes and ovaries

What is low testosterone?

The human body functions within a relatively narrow range of normal; when chemicals such as hormones fall outside those normal levels, there can be consequences that affect the body at a cellular, organ, or systemic (body-wide) level.

Blood tests used to measure testosterone are usually performed in the morning. Testosterone sampling is difficult since the levels normally tend to bounce around a fair bit during the course of the day. The normal value for total testosterone in males is 270-1070 ng/dl. However, this depends to some extent on the individual laboratory being used, and the range can vary as a result. In women, there is debate about the accuracy of testosterone measurements, because the circulating values are so much lower than in males and are harder to accurately measure.

As people age, the amount of testosterone the body produces gradually falls in both men and women. Free testosterone levels can be measured and normal levels depend upon an individual's age. Interestingly, menopause itself does not seem to play a role in a reduction of testosterone levels in women beyond that of advancing age.

Medically Reviewed by a Doctor on 9/12/2012

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