Test Information

This test was developed and its performance characteristics determined by Marshfield Labs.  It has not been cleared or approved by the US Food and Drug Administration.  This test is used for clinical purposes.  It should not be regarded as investigational or for research.
 

Most circulating testosterone is bound to sex hormone-binding globulin (SHBG), which in males also is called testosterone-binding globulin.  A lesser fraction is albumin bound and a small proportion exists as free hormone.  It was initially understood that only the free testosterone was thought to be the biologically active component.  However, testosterone weakly bound to serum albumin which dissociates freely in the capillary bed, becomes readily available for tissue uptake.  Therefore, all non-SHBG-bound testosterone is considered bioavailable.

Male testosterone levels decrease with age; it is generally a gradual decrease in testosterone levels and not a sudden loss of reproductive capability.  Changes seen with lower testosterone with aging include decrease in bone mass, cognitive function, erectile function, libido, muscle strength and sense of wellbeing.

Low testosterone concentrations can be caused by testicular failure (primary hypogonadism) or inadequate stimulation by pituitary gonadotropins (secondary hypogonadism).  Hypogonadism often has high SHBG levels, therefore, the measurement of free or bioavailable testosterone has been recommended when total testosterone levels are normal in men with symptoms of androgen deficiency.

In females, ovary and adrenal glands produce some testosterone, but the majority of the testosterone in women is derived from the peripheral conversion of other steroids.  Often, the first sign of testosterone excess in women is the development of male pattern hair growth, which is referred to as hirsutism.  It has been observed that some women experience hair growth similar to that caused by increased testosterone due to racial or genetic causes and not due to excessive androgens.  Measurement of the testosterone may help to distinguish racial or genetic causes of hirsutism from the abnormal pathology, particularly in women with mixed ethnic backgrounds.

Testosterone secretion is episodic.  Serum testosterone levels in adult males peak in the early morning (7 am), decreasing about 25% to the evening (minimum at about 8 pm).  Levels increase after exercise and decrease after immobilization and after glucose load.