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. The pathogenesis of rickets and osteomalacia in familial hypophosphataemia. Calcium and/or phosphate tolerance tests were performed on patients with familial hypophosphataemia, normal control subjects, and patients with vitamin D deficient osteomalacia.Intestinal calcium absorption was similar in patients with familial hypophosphataemia and control subjects. The phosphate tolerance test, which is known to be 'flat' in patients with familial hypophospbataemia, was normal in patients with vitamin D deficient osteomalacia.These findings suggest that rickets and osteomalacia in familial and some cases of non-familial hypophosphataemia are unrelated to abnormal metabolism of vitamin D. This hypothesis is supported by the fact that intestinal calcium absorption as measured by calcium tolerance test is normal in familial hypophosphataemia.It is suggested that the primary abnormality in familial hypophosphataemia is a partial metabolic block in the intestinal absorption and renal tubular reabsorption of phosphate. There are two main theories to account for the low plasma phosphate found in familial and nonfamilial hypophosphataemia. The first is that it is due to a primary defect in renal tubular reabsorption (Dent, 1952;Fanconi and Girardet, 1952) In order to verify or disprove this hypothesis, intestinal calcium absorption was assessed in patients with familial hypophosphataemia and normal control subjects by means of a calcium tolerance test. The calcium tolerance curves were similar in normal subjects and in patients with familial hypophosphataemia, suggesting that intestinal calcium absorption was normal in this disorder.

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