A rare case of adult hypophosphatasia masked with primary hyperparathyroidism
Authors: Mutlu Ümmü , Sarıbeyliler Göktuğ , Hacişahinoğulları Hülya , Yalın Işcan Ahmet , Aksakal Nihat , Yenidünya Yalın Gülşah , Selçukbiricik Özlem Soyluk , Kubat Üzüm Ayşe , Gül Nurdan 
Published: January 2023

Abstract

Introduction
Hypophosphatasia (HPP) is a rare inherited disorder of bone and mineral metabolism due to mutations in the Alkaline Phosphatase-Liver (ALPL) gene encoding the tissue nonspecific alkaline phosphatase (TNSALP). Clinical features of HPP in adults include early loss of teeth, muscle weakness, musculoskeletal pain, osteoporosis and fragility fractures with delayed healing. The most typical biochemical manifestation of the disease is persistently low level of alkaline phosphatase (ALP). Primary hyperparathyroidism (PHPT) is a relatively common endocrine disorder leading to increase in bone turnover markers including ALP. Coexistence of HPP and PHPT is a very rare condition. Herein we present an adult HPP patient who presented with primary hyperparathyroidism and normal serum ALP levels, which decreased significantly after parathyroidectomy.

Clinical Case
A 33-year-old female with a past medical history of Hashimoto's thyroiditis and acne rosacea admitted to our outpatient clinic because of gradually increasing bone pain, fatigue and weakness. She had early teeth loss and dental problems, but she denied history of fractures and kidney stones. Her family history was remarkable for nephrolithiasis in both parents.
On physical examination the patient had facial rash and telangiectasia. She also had teeth loss and many dental cavities. Her vital signs were within normal limits.
In biochemical investigations creatinine was 0.45 mg/dL (0.7–1.4), ALP 81 U/L (35–105), PTH 531 pg/mL (15–65), calcium 10.84 mg/dL (8.5–10.5), albumin 4.9 g/dL (3.2–5.5), phosphorus 2.25 mg/dL (2.7–4.5), Vitamin D 5 ng/mL and urine calcium excretion 613 mg/day.
Laboratory results were compatible with normocalcemic hyperparathyroidism and severe vitamin D deficiency.
Bone mineral density of the lumbar spine and the femoral neck revealed osteopenia and 1/3 distal radius revealed osteoporosis. There was no evidence of nephrolithiasis and nephrocalcinosis on abdominal ultrasound. Neck ultrasound and CT scan and Tc-99m MIBI scanning of the patient revealed a parathyroid adenoma in the posteroinferior of thyroid gland's left lobe. A successful parathyroidectomy was performed. During follow-up the patient's serum ALP levels decreased significantly to 29 U/L with a repeat being 26 U/L. Bone-specific ALP was 4.2 μg/L (3–14 μg/L). Urine phosphoethanolamine was elevated to 68.6 μmol/g.Cr (0–48), which supported the diagnosis of HPP. Genetic analysis was performed and heterozygous p.N190del (c.568_570delAAC/c.566_568delACA) mutation was detected. This mutation was previously reported as pathogenic and has been associated with adult hypophosphatasia.

Conclusion
Although persistently low level of ALP is characteristic of HPP, disorders that increase bone turnover may result in an increase in serum ALP levels, which may mask underlying HPP.
Acknowledgements We are thankful to Alexion Pharmaceuticals, Inc for providing fund for ALP substrates and genetic analysis of the patient.