Rickets or osteomalacia of variable degrees is a common manifestation of all the diverse conditions associated with renal tubular acidosis. Some of the rachitic disorders are life threatening, such as vitamin D–resistant rickets and osteomalacia due to proximal and distal tubular defects (see Plate 3-17). Other disorders are mild and, in some cases, do not require prolonged treatment. These include the genetically determined, sometimes self-limiting, Butler-Albright syndrome and disorders associated with altered globulin states or hyperthyroidism.
The mechanisms by which renal tubular acidosis contributes to the development of rickets and osteomalacia are not completely understood. Chronic acidosis alone can deplete the bones of calcium and phosphate but is considered to be, at most, a minor mechanism. Many investigators believe that calcium, as well as sodium and potassium, is lost in the urine as fixed base due to a failure to substitute hydrogen or to reabsorb bicarbonate (in types I and II). In some of the syndromes, the %TRP appears to be diminished (either because of a primary renal defect or as a result of secondary hyperparathyroidism and the action of increased concentrations of PTH on the reabsorptive mechanism for phosphate in the renal tubule; or, if found, an elevated FGF-23). The combination of diminished serum calcium and phosphate levels results in rachitic and/or osteomalacic findings that are identical to those seen with other syndromes yet are clearly refractory to even very large doses of vitamin D.
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