Low Serum Alkaline Phosphatase in an Adult: An Incidental Finding or a Case of Hypophosphatasia?

Presentation Number: SUN 301
Date of Presentation: April 2nd, 2017

Whitney Adair*1 and Yin Htwe Oo2
1Dallas VAMC, Dallas, TX, 2Dallas VAMC, Grand Prairie, TX



Hypophosphatasia is the autosomal dominant or recessive inborn error of metabolism with loss‑of‑function mutations in tissue-nonspecific isoenzyme of alkaline phosphatase which eventually leads to defective mineralization of the dentition and skeleton resulting in tooth loss and osteomalacia. Presentations can be varied from severe to nearly asymptomatic.  Here we presented a case of hypophosphatasia in an elderly man presented for thyroid nodule but noted to have asymptomatic persistently low alkaline phosphatase.

Clinical case

A 73 years-old Caucasian man was noted to have persistently low alkaline phosphatase at the range of 17-23 U/L (ref 40-129 U/L) since 2004 when he came in for evaluation of euthyroid multinodular goiter. He has normal level of vitamin D, calcium and phosphate. Alkaline phosphatase isoenzyme confirmed low bone specific alkaline phosphatase. He has no premature tooth loss of either primary or adult teeth. He denies history of multiple fractures but at age 61 he had a fall and X-ray of right ankle showed old fracture which he attributed to a twisted ankle playing football in his teens. Family history is significant only for fracture ankle in one of his sisters at age 63 and fracture of femur in his father at age 94. His physical exam is unremarkable with no khyphoscoliosis and dental caries. Pyridoxal 5ʹ‑phosphate was high at 200 ng/ml (ref 2.1-21.7). Bone density scan showed osteopenia with T score of -1.5 at left femoral neck and – 1.2 at left forearm. Lumbar area is excluded due to degenerative changes. Genetic testing confirmed mutation at tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP).

Clinical lesson

Onset and severity of hypophosphatasia can be varied. Diagnosis can be missed in an individual with late onset presentation. The existence of hypophosphatasia should be considered in a patient with low serum alkaline phosphatase activity for age and sex. Accurate diagnosis is necessary to avoid using anti-resorptive therapy for osteoporosis in asymptomatic hypophosphatasia patients.


Nothing to Disclose: WA, YHO