Studies were conducted in a patient with idiopathic hypophosphatemic
osteomalacia to delineate the roles of
parathyroid hormone (PTH),
vitamin D and renal tubular function. A 43-year-old woman presented with progressive skeletal pains resulting in severe incapacity. Workup revealed:
hypophosphatemia with a low tubular maximal
phosphate reabsorption per glomerular filtrate (
TmP/GFR) of 1.05 mg/dl, normocalcemia, hypocalciuria, elevated
alkaline phosphatase and glycinuria. PTH and urinary
cyclic AMP (UcAMP) were normal, while
calcitriol was low.
Renal tubular acidosis or other transport defects were not present and no
tumor was found. Biopsy was diagnostic for
osteomalacia, and the patient responded to 1-alpha OHD3 and
phosphate therapy.
Hyperparathyroidism was ruled out by 1) normocalcemia persisting after 1-alpha OHD3 and
calcium loading and 2) normal PTH and UcAMP challenged by
phosphate supplements. Combined
calcium and 1-alpha OHD3 administration resulted in
hypercalciuria, decreased UcAMP and increased, but not corrected,
TmP/GFR. These findings suggest that the
osteomalacia was due to
hypophosphatemia caused by a renal leak. PTH is only contributory to the
phosphaturia. Low
calcitriol level contributes to the
osteomalacia directly and indirectly through impaired
mineral absorption and, therefore, is also responsible for the hypocalciuria.