Upon delivery to the target cell, T4 is deiodinated to T3 at the cell surface. T3 is the biologically active form of the thyroid hormone and more readily enters the target cell. Back to Top Total Bilirubin Bilirubin is the final breakdown product of heme. The major sites of bilirubin formation are the spleen and the liver. The remaining bilirubin is largely formed from hepatic cytochromes.
Indirect bilirubin is converted into direct bilirubin by the enzyme glucuronyl transferase.
Direct bilirubin is transported to the biliary system and then to the small intestine where the bilirubin is reduced to urobilinogen. Urobilinogen is excreted in feces, urine, and bile. Total Protein A total protein assay measures the amount of proteins found in the plasma. The two major protein components are albumin and globulins. Plasma proteins play a major role in maintaining colloid osmotic pressure, and are a source of amino acids.