Hungry Bone Syndrome (HBS)

Know who is at risk before the neck case - Incidence is roughly 4% to 13% after parathyroidectomy for primary hyperparathyroidism but 20% to 70% after parathyroidectomy for secondary hyperparathyroidism, and about 39% after thyroidectomy for thyrotoxicosis. Preoperative markers that correlate: elevated PTH (300 to 400 pg/mL in primary disease versus 700 to 1,000 in secondary), elevated alkaline phosphatase, elevated BUN, high BMI, larger resected glands, and radiographic bone disease such as brown tumors, fractures, or osteitis fibrosa cystica. No PTH threshold has been validated as a cutoff.

The timing is postoperative, not intraoperative - By definition the calcium nadir persists past day four, so the exposure is in PACU, on the floor, and at any return trip to the OR, not during the index case. Hand off the calcium trend explicitly rather than assuming the surgical team owns it.

Airway is the emergency - Hypocalcemia causes laryngospasm. In a fresh post-thyroidectomy or post-parathyroidectomy neck it sits alongside two other causes of stridor: a compressive hematoma (about 0.6% incidence, compressing trachea and producing venous congestion of airway structures) and recurrent laryngeal nerve injury, where unilateral damage gives hoarseness and bilateral damage gives stridor with airway occlusion. A hematoma needs immediate wound opening and evacuation, and intubation should not be delayed. Sort the cause fast, because the treatments are not interchangeable.

Get a 12-lead and watch the QTc - Hypocalcemia produces QTc prolongation, arrhythmias, and cardiomyopathy. A QTc change is one of the three triggers for IV calcium.

Neuromuscular signs are the bedside tell - Perioral, hand, and foot paresthesias, carpopedal spasm, tetany, seizures, and positive Trousseau or Chvostek signs. In a patient emerging from anesthesia, tetany can be mistaken for shivering or residual blockade; check a Trousseau or Chvostek sign and send an ionized calcium before you treat it as either.

When to give IV calcium, and how - Calcium under 7.6 mg/dL (1.9 mmol/L), symptoms, or ECG changes. Start with 10% calcium gluconate 10 to 20 mL in 50 to 100 mL of D5W over 5 to 10 minutes, about 100 to 200 mg elemental calcium, then run an infusion. A convenient mix is 100 mL of 10% calcium gluconate in 1 L D5W, roughly 1 mg/mL elemental calcium, started near 50 mL/hr and titrated to a target of 0.5 to 1.5 mg elemental calcium/kg/hr. Recheck calcium, phosphorus, and magnesium every 4 to 6 hours.

Gluconate versus chloride - A gram of calcium chloride carries three times the elemental calcium of a gram of gluconate, but chloride has higher osmolality, is far more damaging on extravasation, and needs central access. Gluconate is the peripheral-line and floor choice.

Replete magnesium, leave phosphate alone - Persistent hypomagnesemia blunts PTH's effect and produces a functional hypoparathyroid state, so calcium replacement will fail until magnesium is corrected. Phosphate is the opposite: it is low, but repleting it precipitates with calcium and drives serum calcium lower still.

Calcitriol lags by days - Active vitamin D, calcitriol 0.25 to 1 mcg/day, runs alongside calcium and magnesium, but several days pass before it moves the serum calcium. A flat calcium at 24 hours is not treatment failure.

Oral requirements are enormous and variable - Once they can swallow, add oral calcium. Carbonate delivers 400 mg elemental calcium per gram versus 211 mg for citrate, so carbonate means fewer pills; citrate does not need an acidic stomach, making it the choice on a PPI or H2 blocker, after gastric bypass, or in the elderly. Reported daily requirements have ranged from 800 mg to 36 g of elemental calcium.

Prevention is not settled - Preoperative bisphosphonates, such as zoledronate 4 mg given 24 to 48 hours before parathyroidectomy in one small retrospective series, shortened length of stay and did not increase HBS risk. The evidence is retrospective and underpowered, so treat it as a surgeon-driven decision rather than a standard.

Hungry bone syndrome (HBS) is profound, prolonged hypocalcemia that follows the abrupt withdrawal of a chronic bone-resorbing stimulus, most often parathyroidectomy for primary or secondary hyperparathyroidism, total thyroidectomy for thyrotoxicosis, and occasionally osteoblastic metastases from prostate cancer. Years of continuous parathyroid hormone (PTH) exposure raise RANK-L and suppress osteoprotegerin, holding bone in net resorption. Remove the source and that reverses: markers of bone formation rise, markers of resorption fall, and the skeleton pulls calcium and phosphate out of the serum faster than gut and kidney can replace them.

Most working definitions require calcium below 8.4 mg/dL persisting beyond four postoperative days, with hypophosphatemia, often hypomagnesemia and hypocalciuria, and a normal PTH. That normal PTH is the discriminator from surgical hypoparathyroidism, where PTH is low and phosphate is high.