A 50-year-old man with polycystic kidney disease comes to the office due to constant, deep pain in his shoulders, arms, and legs. Medical history includes long-standing hypertension treated with ramipril. Blood pressure is 150/85 mm Hg and pulse is 78/min. Cardiopulmonary examination is normal. Abdominal examination shows large, palpable renal masses. Trace bilateral lower-extremity edema is present. Laboratory results from 2 years ago showed a blood urea nitrogen level of 25 mg/dL and a creatinine level of 2.3 mg/dL. Current laboratory results are as follows: Which of the following metabolic states is most likely present in this patient?

Question

A 50-year-old man with polycystic kidney disease comes to the office due to constant, deep pain in his shoulders, arms, and legs. Medical history includes long-standing hypertension treated with ramipril. Blood pressure is 150/85 mm Hg and pulse is 78/min. Cardiopulmonary examination is normal. Abdominal examination shows large, palpable renal masses. Trace bilateral lower-extremity edema is present. Laboratory results from 2 years ago showed a blood urea nitrogen level of 25 mg/dL and a creatinine level of 2.3 mg/dL. Current laboratory results are as follows:

Which of the following metabolic states is most likely present in this patient?

Quizplus · Accepted Answer

11ec160b_3a30_073a_9249_5946957234b4_MD0001_00 This patient's presentation is consistent with metabolic bone disease due to chronic kidney disease (CKD). CKD decreases the glomerular filtration rate (GFR), which decreases the filtered phosphate load and causes elevated serum phosphate levels. Hyperphosphatemia reduces serum free calcium and stimulates osteocytes and osteoclasts to release fibroblast growth factor-23 (FGF-23), a circulating peptide that decreases proximal tubule phosphate reabsorption. Elevated levels of phosphate and FGF-23 also reduce calcitriol synthesis by inhibiting the proximal tubular expression of 1-alpha-hydroxylase, resulting in decreased intestinal calcium and phosphate absorption. This worsens hypocalcemia but does not significantly improve hyperphosphatemia due to the low GFR, which is the limiting factor for phosphate excretion in patients with advanced CKD. Hypocalcemia and hyperphosphatemia also increase parathyroid hormone (PTH) secretion, which stimulates osteoclasts to increase bone turnover. Long-term elevation in PTH (secondary hyperparathyroidism) can eventually lead to friable bones and osteitis fibrosa. Affected patients can develop weakness, bone pain, and fractures. (Choice A) Primary hyperparathyroidism causes hypercalcemia and decreased serum phosphate due to inappropriately elevated PTH. Renal synthesis of calcitriol is also increased by PTH. In contrast, calcitriol levels remain low in patients with CKD due to the reduction in renal mass and the inhibitory effects of FGF-23 and phosphate. (Choice B) Vitamin D toxicity raises calcitriol levels, which increases calcium and phosphate absorption causing hypercalcemia and hyperphosphatemia. Hypercalcemia inhibits PTH release, lowering serum PTH. (Choice D) Vitamin D deficiency in patients with normally functioning kidneys decreases intestinal calcium and phosphate absorption, leading to lower serum phosphate and calcium levels. The resulting hypocalcemia stimulates PTH release from the parathyroid glands. (Choice E) Primary hypoparathyroidism causes hypocalcemia and hyperphosphatemia due to decreased PTH. Calcitriol levels can also be low due to decreased PTH-mediated stimulation of renal 1-alpha-hydroxylase. Educational objective: Chronic kidney disease causes disordered mineralization and bone metabolism that usually presents with hyperphosphatemia, secondary hyperparathyroidism, and decreased calcitriol levels. Patients can be asymptomatic or develop weakness, bone pain, and fractures. __________ References: Chronic kidney disease-mineral and bone disorders (CKD-MBDs): what the endocrinologist needs to know. (http://www.ncbi.nlm.nih.gov/pubmed/24325991)

A 50-Year-Old Man with Polycystic Kidney Disease Comes to the Office