Medical Biochemistry: The Big Picture by Lee W. Janson Marc E. Tischler

Author:Lee W. Janson, Marc E. Tischler
Language: eng
Format: epub
ISBN: 9780071637923
Publisher: McGraw-Hill Companies, Inc.

On the basis of various mechanisms involved in the formation of bone matrix, a number of chemical markers have been characterized, which allow clinicians to measure and monitor bone formation and breakdown. Calcium can be measured as either total calcium or ionized calcium, with calcium bound to serum proteins (e.g., serum albumin) affecting the measurement of calcium freely available to the body. Abnormal levels of albumin (hypoalbuminemia or hyperalbuminemia) will alter the amount of bound calcium, and therefore, albumin levels are often tested as well, so a corrected calcium level can be calculated. Vitamin D levels, specifically the active 1,25(OH)2 D3 (calcitriol) form, will also impact on calcium metabolism and may provide insight into a number of related disease states. Phosphate levels are also important in relation to the binding and excretion of calcium and are, thus, often measured. The important role of PTH in calcium, phosphate, and vitamin D regulation and particular diseases may also prompt a clinician to order measurement of its level. Calcitonin’s importance in calcium regulation usually precludes its measurement, except as a tumor marker for medullary thyroid adenocarcinoma. Finally, alkaline phosphatase, the enzyme that promotes mineralization of newly forming bone, is sometimes measured in patients suffering from bone disorders. As alkaline phosphatase has three different isoforms [intestinal, placental, and nonspecific (e.g., liver/bone/kidney)] and is found in all tissues in the human body, particular isoenzymes need to be isolated and their separated level(s) carefully considered. Alkaline phosphatase is often measured in conditions such as Paget’s disease of bone, osteosarcoma, cancers that have metastasized to bone, bone fractures, osteomalacia (rickets), achondroplasia, congenital hypothyroidism (previously known as cretinism), renal osteodystrophy, hypophosphatasia, osteoporosis/estrogen use, and/or vitamin D deficiency. However, alkaline phosphatase levels are more often used to measure blockage of liver bile ducts than for general bone and calcium studies.

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