Examkin

Question

Difficulty: MediumEndocrinology and Metabolism

A 2828-year-old man comes to the clinic for a routine wellness examination. He has no significant past medical history and takes no medications. He has no symptoms of fatigue, bone pain, abdominal pain, or polyuria. Family history is notable for mild hypercalcemia in his father, who has never required medical or surgical intervention. Vital signs are within normal limits. Physical examination is unremarkable. Laboratory testing reveals the following:

Serum calcium: 11.3 mg/dL11.3\ \text{mg/dL} (normal: 8.4–10.2 mg/dL8.4\text{--}10.2\ \text{mg/dL})
Serum phosphorus: 2.8 mg/dL2.8\ \text{mg/dL} (normal: 2.7–4.5 mg/dL2.7\text{--}4.5\ \text{mg/dL})
Serum creatinine: 0.9 mg/dL0.9\ \text{mg/dL}
Serum intact parathyroid hormone: 58 pg/mL58\ \text{pg/mL} (normal: 10–65 pg/mL10\text{--}65\ \text{pg/mL})
Serum 25-hydroxyvitamin D: 34 ng/mL34\ \text{ng/mL} (normal: 20–50 ng/mL20\text{--}50\ \text{ng/mL})

Based on the information provided, which of the following is the most appropriate next step in the evaluation of this patient?

  1. A
    Technetium-99m sestamibi parathyroid scan
  2. B
    Measurement of serum parathyroid hormone-related peptide
  3. Measurement of 24-hour urinary calcium excretionAnswer
  4. D
    Initiation of oral bisphosphonate therapy
  5. E
    Surgical exploration of the neck

Answer

Measurement of 24-hour urinary calcium excretion
This patient presents with mild, asymptomatic hypercalcemia and an inappropriately normal intact parathyroid hormone (PTH) level. In the presence of hypercalcemia, a normal PTH level is not truly normal; it indicates a failure of the parathyroid glands to adequately suppress hormone secretion. This presentation (PTH-dependent hypercalcemia) is characteristic of primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH). The patient's family history of untreated hypercalcemia strongly suggests FHH, an autosomal dominant disorder caused by an inactivating mutation in the calcium-sensing receptor (CaSR) in the parathyroid glands and kidneys. To distinguish between primary hyperparathyroidism and FHH, the most appropriate next step is measurement of 24-hour urinary calcium excretion. Patients with FHH typically have hypocalciuria (urine calcium-to-creatinine clearance ratio <0.01< 0.01), whereas those with primary hyperparathyroidism usually have normal or elevated urinary calcium.

Step-by-Step Solution

1
Analyze the patient's laboratory findings.
The patient has hypercalcemia with an inappropriately normal intact PTH level.
This establishes the diagnosis of PTH-dependent hypercalcemia, which is primarily caused by either primary hyperparathyroidism or familial hypocalciuric hypercalcemia (FHH).
2
Incorporate the clinical and family history into the diagnostic reasoning.
The patient is completely asymptomatic, and his father has a history of untreated mild hypercalcemia.
This highly suggestive family history points toward FHH, an autosomal dominant benign disorder of the calcium-sensing receptor.
3
Identify the definitive test to differentiate the remaining diagnostic possibilities.
A 24-hour urine calcium collection must be ordered.
FHH causes increased renal calcium reabsorption, resulting in a low urine calcium level (urine calcium-to-creatinine clearance ratio < 0.01). Primary hyperparathyroidism generally presents with normal or elevated urinary calcium.

Key Concept

Patients with PTH-dependent hypercalcemia (elevated or inappropriately normal PTH) must undergo 24-hour urinary calcium measurement to differentiate primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH) before any imaging or surgical interventions are considered.
Rate this question

Topics

Endocrinology and Metabolism

More questions from this topic