Hungry Bone Syndrome

Hungry bone syndrome is a profound, prolonged hypocalcaemia occurring after correction of long-standing hyperparathyroidism, most often following parathyroidectomy, and occasionally after treatment of severe thyrotoxicosis or correction of severe vitamin D deficiency.

The mechanism is a sudden reversal of bone turnover. Chronically elevated PTH drives high bone resorption; when it is abruptly removed, bone formation continues unopposed and the skeleton rapidly takes up calcium, phosphate and magnesium from the circulation.

The result is a hypocalcaemia that is both deeper and far more prolonged than the transient hypocalcaemia of routine post-thyroidectomy hypoparathyroidism, sometimes lasting weeks to months and requiring very large replacement doses.

Distinguishing it from post-surgical hypoparathyroidism matters, because the two look similar initially but differ in phosphate: hungry bone syndrome lowers phosphate and magnesium alongside calcium, whereas hypoparathyroidism raises phosphate.

• Low calcium, low phosphate and low magnesium following parathyroid or thyroid surgery is the characteristic combination and is the key discriminator from post-surgical hypoparathyroidism, in which phosphate is high
• PTH is normal or raised, reflecting an appropriate response to hypocalcaemia in remaining parathyroid tissue, in contrast to the low PTH of surgical hypoparathyroidism
• Alkaline phosphatase is typically markedly raised, reflecting the high bone turnover state, and a high preoperative alkaline phosphatase is one of the best predictors of who will develop the syndrome
• Risk factors worth identifying preoperatively: large adenoma, markedly raised preoperative PTH and calcium, radiological evidence of bone disease, older age, and higher preoperative alkaline phosphatase
• Monitor calcium at least daily, and more frequently in the first 48 hours, in any at-risk patient after parathyroidectomy
• ECG for QT prolongation in significant hypocalcaemia

The defining feature of management is scale and duration: replacement requirements are often far higher and far longer than clinicians anticipate, and under-treatment is the usual error.

• Intravenous calcium gluconate for symptomatic or severe hypocalcaemia, often as a continuous infusion rather than intermittent boluses given the persistent demand
• High-dose oral calcium, frequently several grams of elemental calcium daily in divided doses
• Activated vitamin D (calcitriol or alfacalcidol) in substantial doses, to maximise intestinal calcium absorption
• Magnesium replacement, which is essential and frequently overlooked; persistent hypomagnesaemia will prevent calcium correction regardless of how much calcium is given
• Phosphate is usually left alone and allowed to recover, since giving phosphate will bind calcium and worsen the hypocalcaemia
• Monitor calcium, phosphate and magnesium closely, initially at least daily, tapering the monitoring interval only as levels stabilise
• Requirements fall gradually over weeks to months as the skeleton remineralises, so doses must be actively reduced to avoid eventual hypercalcaemia, patients discharged on high doses need scheduled biochemical review rather than an open-ended prescription

Prevention: identifying high-risk patients preoperatively, correcting vitamin D deficiency before surgery, and planning postoperative monitoring accordingly.

Referral: endocrinology alongside the operating surgical team for all cases, given the complexity and duration of replacement.