Bilirubin is a breakdown product of hemoglobin. Red blood cells contain hemoglobin, which is broken down into the components "heme" and "globin." Heme is converted into bilirubin. Albumin carries bilirubin to the liver, where most of it is conjugated with a glucuronide before it is excreted in the bile. Conjugated bilirubin is called "direct bilirubin;" unconjugated bilirubin is called "indirect bilirubin." Total bilirubin equals direct bilirubin plus indirect bilirubin.
It is suggested that bioavailable testosterone represents the fraction of circulating testosterone that readily enters cells and better reflects the bioactivity of testosterone than does the simple measurement of serum total testosterone. Also, varying levels of SHBG can result in inaccurate measurements of bioavailable testosterone. Decreased SHBG levels can be seen in obesity, hypothyroidism , androgen use, and nephritic syndrome (a form of kidney disease ). Increased levels are seen in cirrhosis , hyperthyroidism , and estrogen use. In these situations, measurement of free testosterone may be more useful.