Hypercalcemia, hypercalciuria, and reduced renal function may complicate treatment. Type II hypophosphatemic rickets responds poorly to treatment, and phosphate replacement is given alone to minimize the degree of hypophosphatemia. In adults with oncogenic rickets, dramatic improvement has followed removal of the small cell mesenchymal tumor that produces a humoral factor that decreases proximal renal tubular resorption of phosphate. Hartnup disease is inherited as an autosomal recessive trait. Consanguinity is a common etiologic factor.
Heterozygotes are phenotypically normal.
Small-intestine malabsorption of tryptophan, phenylalanine, methionine, and other monoaminomonocarboxylic amino acids occurs. Accumulation of unabsorbed amino acids in the GI tract increases their metabolism by bacterial flora. Some tryptophan degradation products, including indoles, kynurenine, and serotonin, are absorbed by the bowel and appear in the urine. Renal amino acid resorption is also defective, causing a generalized aminoaciduria involving all neutral amino acids except proline and hydroxyproline. Conversion of tryptophan to niacinamide is also defective.