Vitamin D dependency type I is a disease characterized by hypocalcemia and rickets in early infancy. We report the case of a female patient diagnosed as having vitamin D dependency type I by the clinical features and the PTH loading test after a low calcium diet. The patient was referred to Osaka University Hospital when she was 2 years 2 months old. She had slight bowing of the legs but no hypocalcemia. Her serum concentrations of 25-hydroxyvitamin D (25OHD) and 1, 25 (OH)
2D were 18.1 ng/ml and 14 pg/ml, respectively. After 3 days on a low calcium diet, the PTH loading test was performed. Neither the low calcium diet nor the injection of PTH increased serum 1, 25 (OH)
2D levels, while serum 1, 25 (OH)
2D levels were significantly increased in normal controls. When active vitamin D metabolites were not administered, serum alkaline phosphatase (ALP) and PTH increased gradually to 1090 U/1 and 1120 pg/ml, respectively. After administration of a physiological dose (0.08μg/kg/day) of 1α-hydroxyvitamin D
3 (1αOHD
3), the serum ALP and PTH levels returned to normal.
The blunted response of serum 1, 25 (OH)
2D concentration to the PTH loading test as well as to the low calcium diet, and the necessity for a physiological dose of 1αOHD
3, suggest that she had a partial defect in renal 25OHD
3-1α-hydroxylase activity.
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