A 15-year-old girl presents with recurrent cramps in both hands for several months. She has also been feeling unusually fatigued lately. She is otherwise well, with no history of significant illnesses or surgeries in the past, and is not on any medications. Her family history is unremarkable. Her menarche was at the age of 13, and her cycles are regular. Her school performance is satisfactory. An electrolyte assay reveals serum total calcium levels of 7.6 mg/dL (normal: 9.5 - 10.4), and phosphate levels of 5.2 mg/dL (normal: 2.5 - 4.5). Serum sodium, potassium, and magnesium levels are within normal parameters. A complete blood count, liver profile (including serum albumin), renal profile, and ECG are found to be normal.
Serum parathyroid hormone (PTH) levels: 160 pg/mL (10 - 65)
Serum 25-OH vitamin D: 45 ng/mL (20 - 100)
Both urinary phosphate excretion and urinary cAMP excretion show an impaired response.
SCr: 0.74 mg/dL (0.6 - 1.2) BUN: 18 mg/dL (7 - 20)