Endocrine Abstracts

Pseudohypoparathyroidism-1b (PHP-1b) is a genetically and biochemically distinct disorder characterised by selective PTH resistance limited to the kidney and lack of Albright Hereditary Osteodystrophy (AHO). Due to calcium mobilization from bone, patients with PHP-1b may remain asymptomatic and undiagnosed for a long time. Coexistent vitamin D deficiency sometime unmasks the clinical presentation of the disease.

A 15-year-old Kuwaiti male presented with short stature, episodes of tetany, carpopedal spasm and arthralgia. His height was >2SD below predicted adult height but bone age was compatible with chronological age. There was no facial dysmorphism, skeletal deformities or features of AHO. Dynamic tests for GH and serial IGF1 was within normal limit. Baseline investigation revealed hypocalcemia, hyperphosphatemia, elevated alkaline phosphatase, low 25(OH) D 10.7 nmol/L (23–113), secondary hyperparathyroidism 11.1 pmol/L (0.7–5.6) and absent hypomagnesemia. Urine cAMP excretion was low normal 2.72Z μmol/24Hr (2–10) in the presence of elevated PTH 13.8 pmol/L. Lumbar spine BMD was very low (T score −3.58) which improved gradually with 1(OH)D, calcium and phosphate binding antacid treatment over next 6 year (final T score −1.2).

The diagnosis of PHP in the presence of vitamin D deficiency is difficult since it may mimics PHP-2. However persistent hyperphosphatemia, elevated PTH and impaired renal cAMP response in spite of normalization of vitamin D status confirmed associated PHP- 1b.