This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today’s case focuses on a 56-year-old woman who reported a sensation of numbness on her mouth and intermittent twitching of her extremities. Physical examination demonstrates an intact incision over the anterior neck without edema or erythema. Which of the following laboratory findings is most likely to be present based on this patient’s clinical presentation? Can you figure it out?
A 56-year-old woman is currently postoperative day two in the surgical intensive care unit post total thyroidectomy for anaplastic thyroid carcinoma. During rounds, the patient reports a sensation of numbness around her lips and mouth and intermittent twitching of her upper and lower extremities. The patient’s temperature is 37°C (98.6°F), pulse is 101/min, respirations are 18/min, blood pressure is 156/67 mmHg, and O2. is 95% on room air. Physical examination demonstrates an intact incision over the anterior neck without edema or erythema. A routine electrocardiogram is demonstrated below. Which of the following laboratory findings is most likely to be present based on this patient’s clinical presentation?

Reproduced from: Wikipedia
Ca2+ | PO43- | PTH | |
A. | ↓ | ↑ | ↓ |
B. | ↓ | ↑ | ↑ |
C. | ↓ | ↓ | ↑ |
D. | ↓ | ↓ | ↓ |
E. | ↑ | ↑ | ↓ |
Scroll down to find the answer!
The correct answer to today’s USMLE® Step 1 Question is…
A. | ↓ | ↑ | ↓ |
Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
The incorrect answers to today’s USMLE® Step 1 Question are…
B. | ↓ | ↑ | ↑ |
Incorrect: Secondary hypoparathyroidism, due to chronic kidney disease, manifests with decreased levels of calcium, increased levels of phosphate, and increased parathyroid hormone. These effects are secondary to the kidney’s inability to respond to parathyroid hormone appropriately.
C. | ↓ | ↓ | ↑ |
Incorrect: Vitamin D deficiency is characterized by decreased calcium, decreased phosphate, and increased parathyroid hormone levels.
D. | ↓ | ↓ | ↓ |
Incorrect: These findings are not consistent with calcium, phosphorus, and parathyroid hormone homeostasis disorders.
E. | ↑ | ↑ | ↓ |
Incorrect: These findings are not consistent with calcium, phosphorus, or parathyroid hormone homeostasis disorders.

Main Explanation
This patient presents after total thyroidectomy with findings of circumoral numbness, tetany, and a prolonged QT interval on routine electrocardiographic screening, consistent with hypocalcemia. In the setting of a recent thyroidectomy, the most likely etiology of hypocalcemia is removal or damage to the parathyroid glands, resulting in hypoparathyroidism. The laboratory findings most reflective of this clinical condition is choice A.
Multiple structures are at risk of injury during thyroidectomy or hemithyroidectomy, including surrounding glands, vasculature, and nerves (e.g., recurrent laryngeal nerves, external branches of the superior laryngeal nerves). Injury to the parathyroid glands can produce transient or permanent hypoparathyroidism. For this reason, many patients undergoing this procedure are empirically prescribed calcium supplementation to avoid complications associated with hypocalcemia.
Symptoms of hypoparathyroidism and associated hypocalcemia include muscle tetany, paresthesias, seizures, and psychiatric symptoms (e.g., emotional instability, anxiety, depression). Moreover, hypocalcemia can cause cardiovascular compromise, resulting in QT interval prolongation on ECG, which can degenerate into torsade de pointes. Physical exam findings include the Chvostek sign and Trousseau sign (carpal spasms triggered by inflation of a sphygmomanometer).
Major Takeaway
Multiple structures are at risk of injury during thyroid surgery, including the surrounding parathyroid glands. Damage to or accidental removal of these glands may result in transient or permanent hypoparathyroidism symptoms and subsequent hypocalcemia.
References
Loyo M, Tufano RP, Gourin CG. National trends in thyroid surgery and the effect of volume on short-term outcomes. Laryngoscope 2013; 123:2056.
Kandil E, Noureldine SI, Abbas A, Tufano RP. The impact of surgical volume on patient outcomes following thyroid surgery. Surgery 2013; 154:1346. _________________________
Want more USMLE® Step 1 practice questions? Try Osmosis today! Access your free trial and find out why millions of current and future clinicians and caregivers love learning with us.

The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB.
Leave a Reply