A 41-year-old woman has hypocalcemia, hyperphosphatemia, and decreased urinary phosphate excretion. Injection of parathyroid hormone (PTH) causes an increase in urinary cyclic adenosine monophosphate (cAMP). The most likely diagnosis is

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[VII B 3 b]. Low blood $\left[\mathrm{Ca}^{2+}\right]$ and high blood [phosphate] are consistent with hypoparathyroidism. Lack of parathyroid hormone (PTH) decreases bone resorption, decreases renal reabsorption of $\mathrm{Ca}^{2+}$, and increases renal reabsorption of phosphate (causing low urinary phosphate). Because the patient responded to exogenous PTH with an increase in urinary cyclic adenosine monophosphate (cAMP), the G protein coupling the PTH receptor to adenylate cyclase is apparently normal. Consequently, pseudohypoparathyroidism is excluded. Vitamin D intoxication would cause hypercalcemia, not hypocalcemia. Vitamin D deficiency would cause hypocalcemia and hypophosphatemia.

A 41-Year-Old Woman Has Hypocalcemia, Hyperphosphatemia, and Decreased Urinary Phosphate D\mathrm{D}D

A 41-Year-Old Woman Has Hypocalcemia, Hyperphosphatemia, and Decreased Urinary Phosphate $\mathrm{D}$