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Difficulty: HardInterpreting Laboratory Results

A 58-year-old man presents to the clinic for evaluation of progressive, diffuse muscle weakness and a dull, aching pain in his lower back, pelvis, and thighs that has worsened over the past six months. He reports difficulty climbing stairs and rising from low chairs. He was diagnosed with celiac disease three years ago but admits to frequently consuming gluten-containing foods due to lifestyle constraints. He has no history of fractures or significant trauma. Vital signs are within normal limits. Physical examination reveals mild tenderness to palpation over the lumbar spine and bilateral proximal muscle weakness in the lower extremities (4/5 strength).

Laboratory results are as follows:

TestResultReference Range
Calcium, total8.2 mg/dL8.6–10.3 mg/dL
Phosphorus2.1 mg/dL2.5–4.5 mg/dL
Alkaline phosphatase385 U/L40–120 U/L
Gamma-glutamyl transferase (GGT)28 U/L9–48 U/L
Parathyroid hormone, intact115 pg/mL15–65 pg/mL
Creatinine0.8 mg/dL0.7–1.3 mg/dL

Which of the following is the most likely diagnosis based on these clinical and laboratory findings?

  1. A
    Paget disease of bone
  2. B
    Primary hyperparathyroidism
  3. OsteomalaciaAnswer
  4. D
    Renal osteodystrophy
  5. E
    Primary biliary cholangitis

Answer

Osteomalacia
The correct answer is Osteomalacia. This patient's presentation of diffuse bone pain, proximal muscle weakness, and a history of poorly compliant celiac disease is highly suspicious for severe vitamin D deficiency leading to osteomalacia. The laboratory findings confirm this diagnosis through a specific multi-step pattern. First, the elevated alkaline phosphatase (ALP) is determined to be of bone origin rather than hepatic origin because the gamma-glutamyl transferase (GGT) is entirely normal. Second, the combination of low-normal calcium, low phosphorus, and significantly elevated parathyroid hormone (PTH) demonstrates secondary hyperparathyroidism. In the setting of celiac-induced malabsorption, the primary insult is vitamin D deficiency, which drastically reduces intestinal calcium and phosphorus absorption. The resulting transient hypocalcemia triggers the parathyroid glands to over-secrete PTH. The elevated PTH acts to normalize serum calcium at the expense of bone demineralization (raising bone ALP) while simultaneously causing renal phosphate wasting (lowering serum phosphorus). This profound defect in bone matrix mineralization manifests pathologically and clinically as osteomalacia.

Step-by-Step Solution

1
Determine the tissue origin of the elevated alkaline phosphatase.
The alkaline phosphatase is determined to be of bone origin.
The gamma-glutamyl transferase (GGT) is within normal limits, ruling out a hepatobiliary source for the elevated alkaline phosphatase.
2
Analyze the relationship between calcium, phosphorus, and parathyroid hormone (PTH).
The pattern indicates secondary hyperparathyroidism.
The low-normal total serum calcium has appropriately triggered a compensatory rise in parathyroid hormone secretion. The high PTH level then acts on the kidneys to increase phosphate excretion, resulting in hypophosphatemia.
3
Synthesize the laboratory pattern with the patient's medical history to establish the final diagnosis.
The underlying cause is severe vitamin D deficiency leading to osteomalacia.
The patient has poorly controlled celiac disease, which causes malabsorption of fat-soluble vitamins (A, D, E, K). The resulting profound vitamin D deficiency prevents adequate bone mineralization (osteomalacia), leading to the patient's progressive bone pain and proximal myopathy, supported by the classic laboratory profile of secondary hyperparathyroidism.

Key Concept

Secondary hyperparathyroidism due to vitamin D deficiency (osteomalacia)
Estimated Time:2m 0s
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Topics

Interpreting Laboratory Results

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