A 65-year-old man is referred to you from his primary care physician (PCP) for evaluation and management of pos-sible osteoporosis. He saw his PCP for evaluation of low back pain. X-rays of the spine showed some degenerative changes in the lumbar spine plus several wedge deformities in the thoracic spine. The patient is a long-time smoker (up to two packs per day) and has two to four glasses of wine with dinner, more on the weekends. He has chronic bronchitis, presumably from smoking, and has been treated on numerous occasions with oral prednisone for exacerba-tions of bronchitis. He is currently on 10 mg/d prednisone. Examination shows kyphosis of the thoracic spine, with some tenderness to fist percussion over the thoracic spine. The dual-energy x-ray absorptiometry (DEXA) measure-ment of the lumbar spine is “within the normal limits,” but the radiologist noted that the reading may be misleading because of degenerative changes. The hip measurement shows a T score (number of standard deviations by which the patient’s measured bone density differs from that of a normal young adult) in the femoral neck of –2.2. What further workup should be considered, and what therapy should be initiated?
Calcium and phosphate, the major mineral constituents of bone, are also two of the most important minerals for general cellular function. Accordingly, the body has evolved complex mechanisms to carefully maintain calcium and phosphate homeostasis (Figure 42–1). Approximately 98% of the 1–2 kg of calcium and 85% of the 1 kg of phosphorus in the human adult are found in bone, the principal reservoir for these minerals. This reservoir is dynamic, with constant remodeling of bone and ready exchange of bone mineral with that in the extracellular fluid. Bone also serves as the principal structural support for the body and provides the space for hematopoiesis. This relationship is more than fortuitous, as elements of the bone marrow affect skeletal processes just as skeletal elements affect hematopoietic processes. During aging and in nutritional diseases such as anorexia nervosa and obesity, fat accumulates in the marrow, suggesting a dynamic interaction between marrow fat and bone. Furthermore, bone has been implicated as an endocrine tissue with release of osteocalcin, which in its uncarboxylated form stimulates insulin secretion and testicular function. Abnormalities in bone mineral homeostasis can lead to a wide variety of cellular dysfunctions (eg, tetany, coma, muscle weakness), disturbances in structural support of the body (eg, osteoporosis with fractures), and loss of hematopoietic capacity (eg, infantile osteopetrosis).
Calcium and phosphate enter the body from the intestine. The average American diet provides 600–1000 mg of calcium per day, of which approximately 100–250 mg is absorbed. This amount represents net absorption, because both absorption (principally in the duodenum and upper jejunum) and secretion (principally in the ileum) occur. The quantity of phosphorus in the American diet is about the same as that of calcium. However, the efficiency of absorption (principally in the jejunum) is greater, ranging from 70% to 90%, depending on intake. In the steady state, renal excretion of calcium and phosphate balances intestinal absorption. In general, more than 98% of filtered calcium and 85% of
Ca, P D(+), PTH (+)
D(+), PTH (+) CT(–)
Serum Ca, P Gut Ca, P D(–)D(–) PTH(+)PTH(–) CT(+) CT(+) FGF23(+)