Low serum alkaline phosphatase activity due to asymptomatic hypophosphatasia in a teenage girl.

OBJECTIVE

The case report details an unusual presentation of a teenage patient with hypophosphatasia.

METHODS

A 17 year-old female patient presented to endocrinology for the evaluation of fatigue and possible adrenal insufficiency. In the course of her clinical evaluation she was noted to have a low serum alkaline phosphatase activity. Relatively few conditions are associated with a low serum alkaline phosphatase including Wilson's disease, hypophosphatasia, pernicious anemia and untreated hypothyroidism.

RESULTS

Laboratory testing for hypothyroidism were unrevealing, as were the results for vitamin B12 and vitamin D. Testing for Wilson's disease revealed a ceruloplasmin concentration of 165 mg/L (Reference Interval, 160-450 mg/L), however sequencing of the ATP7B gene revealed no deleterious mutations. Measurement of serum pyridoxal phosphate and urine phosphoethanolamine for the diagnosis of hypophosphatasia revealed concentrations of 541.5 nmol/L (reference interval: 29.6-295.5) and 707 mmol/mol creatinine (reference interval: <778 mmol/mol creatinine), respectively, consistent with a diagnosis of hypophosphatasia.

CONCLUSIONS

Hypophosphatasia was initially considered an unlikely diagnosis for this patient given her lack of characteristic skeletal abnormalities. This diagnosis of hypophosphatasia in this case was complicated by a serum ceruloplasmin concentration at the lower end of the reference interval leading to the genetic testing for Wilson's disease.