Q&A Medicine>>>>>Endocrine and Metabolic Disorders
Question 4#

A 36-year-old woman with a history of Graves disease presents with perioral numbness and muscle cramps for the last 2 weeks. She underwent a thyroidectomy 2 months ago for a large goiter that was beginning to compress her trachea. She has never had these symptoms before and her family history is insignificant. Physical examination is unremarkable and laboratory studies reveal the following:

• Sodium 138 mEq/L
• Potassium 4.2 mEq/L
• Chloride 101 mEq/L
• Bicarbonate 24 mEq/L
• Blood urea nitrogen 11 mg/dL
• Creatinine 0.9 mg/dL
• Glucose 98 mg/dL (fasting)
• Calcium 6.2 mg/dL
• Phosphorus 5.3 mg/dL

Which of the following is associated with her condition?

A. Peaked T waves on ECG
B. Widening of the QRS complex on ECG
C. Prolongation of the QT interval on ECG
D. Shortened QT interval on ECG

Correct Answer is C

Comment:

Prolongation of the QT interval on ECG. The patient in this question presents with signs and symptoms of hypocalcemia (perioral numbness and muscle cramps). Other signs of hypocalcemia include tetany, carpopedal spasms, and neuromuscular irritability. The patient has primary hypoparathyroidism as evidenced by low serum calcium levels and elevated phosphorus levels. Chronic renal failure can also produce low serum calcium levels and elevated phosphorus levels, but this patient has normal BUN and creatinine levels, so she does not show signs of kidney failure. On ECG, hypocalcemia presents as a prolongation of the QT interval. (A) Peaked T waves on ECG are seen with hyperkalemia. (B) Widening of the QRS complex is also seen with hyperkalemia, but can be seen with bundle branch block as well. (D) Shortened QT interval on ECG is seen with hypercalcemia.