A 30-year-old woman has had diarrhea and weight loss for the past several months. She also has diffuse bone pain and generalized weakness. The patient has a history of primary hypothyroidism for which she takes levothyroxine. She is 162.5 cm (5 ft 4 in) tall and weighs 45 kg (99.2 lb) . BMI is 17 kg/m². On physical examination, the abdomen is soft and nontender. Initial laboratory evaluation reveals microcytic anemia, hypoalbuminemia, normal magnesium level, and normal serum TSH concentration. The patient is found to have positive anti-tissue transglutaminase IgA antibodies. Which of the following sets of additional laboratory findings are most likely present in this patient?

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This patient with diarrhea, weight loss, and a positive tissue transglutaminase antibody assay has celiac disease, an immune-mediated hypersensitivity to dietary gluten. Celiac disease is characterized by villous atrophy in the small intestine, leading to malabsorption of dietary fats and fat-soluble vitamins (ie, A, D, E, K). The resulting vitamin D deficiency can present as rickets in children and osteomalacia in adults. Vitamin D increases intestinal absorption of calcium and phosphorus; deficiency reduces calcium absorption, which in turn stimulates release of parathyroid hormone (PTH). Vitamin D also directly inhibits PTH release, and therefore vitamin D deficiency facilitates a significant rise in PTH (secondary hyperparathyroidism). PTH induces release of calcium and phosphorus from bones, leading to decreased bone mineralization. (Although most phosphorus in the body is in the form of hydroxyapatite in bone, circulating phosphorus is primarily in the form of phosphate/phosphoric acids.) Typical laboratory findings in vitamin D deficiency include: low 25-hydroxyvitamin D, which reflects total body vitamin D stores (PTH stimulates renal conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D; therefore, 1,25-dihydroxyvitamin D may remain within laboratory norms). elevated PTH. increased alkaline phosphatase, reflecting increased bone turnover. low serum phosphorus, due to decreased intestinal absorption and increased PTH-mediated renal excretion. PTH may initially maintain normal serum calcium levels by reducing urinary calcium excretion. However, hypocalcemia may eventually develop as bone stores are depleted in later or more severe cases. (Choice A) Primary hyperparathyroidism presents with elevated PTH, mild hypercalcemia, and low serum phosphorus (due to increased renal excretion) but is usually due to a parathyroid adenoma and is not associated with celiac disease. (Choice C) In hypoparathyroidism, low serum PTH is accompanied by hypocalcemia and an increase in serum phosphorus. Hypoparathyroidism can be caused by severe hypomagnesemia, which can be seen with prolonged diarrhea, but this patient's magnesium level is normal. (Choices D and E) Chronic kidney disease causes hyperphosphatemia due to decreased filtration and excretion of phosphate. Concurrently, there is decreased renal production of 1,25-dihydroxyvitamin D (the activated form) due to inadequate renal metabolic activity and suppression of 1-alpha-hydroxylase by hyperphosphatemia. Hypocalcemia may be present, although increased PTH sometimes maintains calcium within laboratory norms. Pseudohypoparathyroidism is characterized by resistance to PTH and presents with similar biochemical markers. Educational objective: Malabsorption caused by celiac disease can lead to vitamin D deficiency. Patients have decreased serum phosphorus, increased serum parathyroid hormone (secondary hyperparathyroidism), and low (or normal) serum calcium. __________ References: Osteomalacia as a result of vitamin D deficiency (http://www.ncbi.nlm.nih.gov/pubmed/20511054)

A 30-Year-Old Woman Has Had Diarrhea and Weight Loss for the Past