Low ionized calcium producing neuromuscular irritability; correct magnesium first if low.
Also known as: hypocalcemia, tetany, Chvostek sign, Trousseau sign, low calcium
Overview
Total calcium <8.5 mg/dL (corrected for albumin) or ionized calcium <4.65 mg/dL. Symptoms generally appear with ionized Ca <4.0 mg/dL, but acuity and chronicity matter more than absolute number.
Epidemiology
Common in hospitalized patients — present in up to 88% of ICU patients. Causes vary by setting: post-thyroidectomy in surgical patients, vitamin D deficiency in outpatients, pancreatitis and sepsis in critical care, hypomagnesemia in alcohol use and PPI users.
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Question 1EndocrineEasy
A 48-year-old woman is in the recovery room after total thyroidectomy for multinodular goiter. Six hours postoperatively she develops perioral numbness and tingling in her fingertips. Tapping the facial nerve anterior to the ear produces ipsilateral facial twitching, and inflating a blood pressure cuff above systolic for 3 minutes produces carpal spasm. Which mechanism best explains her symptoms?
ACalcitonin release from surgical manipulation of thyroid C cells
CPostoperative hypothyroidism from removal of thyroid tissue
DHyperventilation-induced respiratory alkalosis
EHypocalcemia from injury or devascularization of the parathyroid glands✓
Why Hypocalcemia from injury or devascularization of the parathyroid glands is correct
Chvostek sign (facial twitch with tapping CN VII) and Trousseau sign (carpal spasm with BP cuff inflation above systolic for 3 minutes) reflect neuromuscular irritability from hypocalcemia
Inadvertent removal, devascularization, or trauma to the parathyroid glands during thyroidectomy is the classic cause of acute postoperative hypocalcemia
Treatment is IV calcium gluconate and oral calcium plus calcitriol; magnesium should also be repleted
Why the others are wrong
Hyperventilation-induced respiratory alkalosis — can lower ionized calcium but does not fit the surgical context as the primary cause
Postoperative hypothyroidism from removal of thyroid tissue — develops over weeks, not hours, and does not cause tetany
Hypomagnesemia impairing parathyroid hormone release — low magnesium does blunt PTH secretion and cause hypocalcemia resistant to calcium alone, but tetany appearing 6 hours after total thyroidectomy is explained by direct parathyroid injury; magnesium is checked and repleted alongside calcium rather than being the mechanism here
Calcitonin release from surgical manipulation of thyroid C cells — parafollicular C-cell calcitonin can lower calcium, but it has little effect on serum calcium in humans (patients with medullary thyroid carcinoma and very high calcitonin levels stay normocalcemic), and a total thyroidectomy removes the C cells; the tetany reflects lost parathyroid function.
Question 2EndocrineMedium
A 44-year-old woman is evaluated on the first postoperative day after total thyroidectomy for multinodular goiter. She reports tingling around her mouth and in her fingertips that began overnight, along with intermittent hand cramping. On examination, inflating a blood pressure cuff above her systolic pressure for 3 minutes reproduces flexion of the wrist and metacarpophalangeal joints with extension of the interphalangeal joints and adduction of the thumb. An ECG shows a prolonged QT interval. Serum magnesium is 2.1 mg/dL. Which of the following is the most likely diagnosis?
AHypocalcemia
BHypothyroidism
CHypomagnesemia
DHypophosphatemia
EHypokalemia
Reveal answer & full explanation
Correct answer: A — Hypocalcemia
AHypocalcemia✓
BHypothyroidism
CHypomagnesemia
DHypophosphatemia
EHypokalemia
Why Hypocalcemia is correct
Total thyroidectomy can injure or devascularize the parathyroid glands, causing transient (or rarely permanent) hypoparathyroidism and acute hypocalcemia, typically within the first 24-72 hours.
The triad here is classic: perioral and acral paresthesias (early neuromuscular irritability), a positive Trousseau sign (carpopedal spasm after 3 minutes of cuff inflation above SBP — more specific than Chvostek), and a prolonged QT on ECG.
Low ionized calcium increases neuromuscular excitability and prolongs the cardiac action potential plateau, lengthening the QT and risking torsades de pointes.
Management: confirm with ionized calcium, check and correct magnesium, and treat symptomatic disease with IV calcium gluconate (calcium chloride only via central line).
Why the others are wrong
Hypophosphatemia — produces muscle weakness, rhabdomyolysis, and altered mentation; hypoparathyroidism actually raises phosphate, and hypophosphatemia does not produce tetany or a positive Trousseau sign.
Hypokalemia — causes muscle weakness, cramps, and ECG changes (U waves, flattened T waves), not the tetany, perioral paresthesias, and positive Trousseau sign seen here.
Hypomagnesemia — can cause neuromuscular irritability and impairs PTH secretion/action (a hypoparathyroid-like state), but her serum magnesium is normal at 2.1 mg/dL, which excludes it here; the defining post-thyroidectomy event is parathyroid injury producing hypocalcemia.
Hypothyroidism — the obvious worry after removing the entire thyroid, and hypothyroidism can cause muscle cramps and QT prolongation, but thyroxine's roughly one-week half-life means hypothyroidism does not develop by the first postoperative day, and it does not produce tetany or a positive Trousseau sign.
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Pseudohypocalcemia from hypoalbuminemia — correct calcium (add 0.8 mg/dL per 1 g/dL albumin deficit) or measure ionized calcium
Pathophysiology
Calcium is regulated by PTH, vitamin D, and calcitonin. PTH increases bone resorption, renal calcium reabsorption, and 1-alpha-hydroxylation of vitamin D. Vitamin D increases intestinal absorption. Hypocalcemia results from failure of any of these mechanisms, increased calcium binding (citrate, phosphate, fat), or shifts in protein/pH (acidosis raises ionized Ca; alkalosis lowers it).
Clinical presentation
Symptoms
Perioral and acral paresthesias (early)
Muscle cramps, carpopedal spasm, tetany
Anxiety, irritability, depression, confusion
Seizure, laryngospasm, bronchospasm (severe)
Symptoms more pronounced with rapid drops and alkalosis
Signs / physical exam
Chvostek sign — twitch of upper lip / nasolabial fold with tapping over facial nerve anterior to ear (low sensitivity; positive in ~10% of normals)
Trousseau sign — carpal spasm after 3 min of BP cuff inflation above SBP (more specific)
Prolonged QTc on ECG; risk of torsades
Stridor, laryngeal spasm in severe disease
Hyperreflexia, papilledema, seizure in extreme cases
Classic findings
Patient on POD 1 after total thyroidectomy with perioral tingling, positive Trousseau, and prolonged QT.
Differential diagnosis
Hypoalbuminemia (pseudohypocalcemia) — Total Ca low but ionized Ca normal; correct for albumin or measure ionized
Hypoparathyroidism — Low PTH, low Ca, high phosphate; postsurgical most common
Pseudohypoparathyroidism — HIGH PTH (resistance), low Ca, high phosphate; Albright osteodystrophy phenotype
Vitamin D deficiency — Low 25-OH vitamin D; elevated PTH (secondary); low/normal Ca, low phosphate
Hypomagnesemia — Mg <1.0; impairs PTH secretion and action; correct first
Nephrolithiasis / nephrocalcinosis from over-replacement
Osteomalacia or rickets (vitamin D deficiency)
Hungry bone syndrome (post-parathyroidectomy)
PANCE pearls
ALWAYS check and correct magnesium first — hypomagnesemia produces hypoparathyroid-like state.
Check ionized calcium in any acute or severe presentation — total calcium misleads in albumin disturbance and alkalosis.
Alkalosis (hyperventilation, blood gas analysis) shifts calcium to protein-bound form — produces tetany at normal total calcium.
Treat acute symptomatic hypocalcemia with IV calcium gluconate; calcium chloride only via central line (vesicant).
Chvostek can be positive in normal people; Trousseau is more specific.
Post-thyroidectomy hypocalcemia is usually transient (recovery in days to weeks) but can be permanent. Monitor closely first 48-72 h.
References
Endocrine Society 2016 — Management of Hypoparathyroidism: Summary Statement and Guidelines (Brandi et al., J Clin Endocrinol Metab 2016)
ESE 2015 — European Society of Endocrinology Clinical Guideline: Treatment of Chronic Hypoparathyroidism (Bollerslev et al., Eur J Endocrinol 2015)
AACE 2003 — American Association of Clinical Endocrinologists Medical Guidelines for Clinical Practice for the Evaluation and Treatment of Hypogonadism — and supplemental electrolyte guidance
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