Hypocalcaemia and hypoparathyroidism — viva

Opening (must-hit)

"I will treat this as an emergency first: she has symptomatic hypocalcaemia with carpopedal spasm, so I will give intravenous 10% calcium gluconate at 0.5 millilitres per kilogram over 5 to 10 minutes slowly with cardiac monitoring. The diagnosis is postsurgical hypoparathyroidism — the undetectable PTH five days after total thyroidectomy confirms it. Once she is stable, I will start oral calcium and calcitriol, check her magnesium, and plan serial calcium monitoring because some of these cases recover and some are permanent." [1][3]

Branch A — Recognition and classification

Examiner: What is the most likely diagnosis and how do you classify it? Candidate: Postsurgical hypoparathyroidism. She has symptomatic hypocalcaemia (ionised 0.75, carpopedal spasm), a high phosphate (2.1), and an undetectable PTH (0.8 pmol/L) five days after total thyroidectomy. The calcium-PTH axis is broken at the source: the parathyroid glands have been removed, devascularised, or stunned during the thyroidectomy, so no PTH reaches the bone and kidney. The classification is hypocalcaemia with low PTH, which is hypoparathyroidism — as opposed to high PTH, which would be vitamin D deficiency or pseudohypoparathyroidism. [3]

Branch B — Emergency management

Examiner: Give me the exact emergency management. Candidate: Intravenous 10% calcium gluconate at 0.5 millilitres per kilogram, maximum 20 millilitres, over 5 to 10 minutes with continuous cardiac monitoring. The critical rule is to give it slowly, because rapid intravenous calcium causes bradycardia, arrhythmia, and can arrest the heart in systole. I use calcium gluconate rather than calcium chloride because it is less vesicant if it extravasates through a peripheral line. After the bolus, I transition to a continuous infusion or oral calcium and calcitriol once she is stable. [1]

Branch C — The magnesium pitfall

Examiner: The calcium rises to 0.9 but she is still symptomatic and the calcium drifts down again over the next few hours. What do you check? Candidate: Magnesium. Severe hypomagnesaemia (below 0.4 millimoles per litre) both suppresses PTH secretion and induces PTH resistance, so the hypocalcaemia will not correct with calcium alone until magnesium is replaced. This is the single most commonly missed reversible factor in refractory hypocalcaemia. I check the level and replace it — intravenously if severe, orally if mild — then recheck the calcium. [3]

Branch D — Chronic management and targets

Examiner: She is now stable. What is the long-term management and what calcium target do you aim for? Candidate: Oral elemental calcium at 30 to 75 milligrams per kilogram per day in divided doses, plus calcitriol at 20 to 60 nanograms per kilogram per day. I use calcitriol — not native vitamin D — because there is no PTH to drive the renal 1-alpha-hydroxylation step. The treatment target is a calcium just below the normal range, not high-normal, because the unprotected kidney (no PTH to reabsorb calcium) develops hypercalciuria, nephrocalcinosis, and nephrolithiasis when the calcium is driven too high. I monitor the urinary calcium-to-creatinine ratio at every visit. [1][7]

Branch E — Transient versus permanent

Examiner: Five days post-op. Will this be permanent? Candidate: Not necessarily. Postsurgical hypoparathyroidism is often transient: the glands may have been stunned rather than removed, and they can recover over weeks to months. The undetectable PTH immediately post-operatively is concerning for a higher risk of permanence, but I cannot be certain at five days. I check serial calcium every 12 hours for the first 48 hours, continue oral calcium and calcitriol, and reassess PTH and calcium at two weeks, six weeks, and three months. If the PTH recovers and the calcium is stable off replacement, it was transient. If not, it is permanent. [1][7]

Branch F — Pseudohypoparathyroidism

Examiner: Suppose the PTH had been 30 pmol/L (high) instead of undetectable. How would that change things? Candidate: That would be pseudohypoparathyroidism — end-organ resistance, not gland failure. The GNAS mutation produces a defective Gs-alpha protein, so the PTH receptor cannot generate the cAMP signal, and the kidney and bone cannot respond despite abundant PTH. The biochemistry looks like hypoparathyroidism (low calcium, high phosphate) but with a high PTH. I would look for the Albright hereditary osteodystrophy phenotype: short stature, round face, brachydactyly, subcutaneous ossifications. The management is still calcium and calcitriol — the same as for hypoparathyroidism — because the goal is to correct the hypocalcaemia, not to supply PTH. [4]

Branch G — PTH 1-34 in refractory disease

Examiner: If she cannot maintain a safe calcium on conventional therapy without hypercalciuria, what is the next step? Candidate: PTH 1-34 under specialist paediatric endocrine care. The Winer studies demonstrated that long-term PTH 1-34 in children maintains calcium in target with lower supplemental calcium and calcitriol doses and reduced urinary calcium, and that twice-daily dosing produces more stable levels than once-daily. It is reserved for refractory disease — where conventional therapy cannot maintain a safe calcium without causing hypercalciuria — and requires close monitoring. [5]

Branch H — Neonatal hypocalcaemia

Examiner: How would your approach differ in a neonate? Candidate: In a neonate, I use ionised calcium rather than total calcium, because albumin and protein binding are unreliable. I classify by timing: early hypocalcaemia (first 72 hours) is common in prematurity, maternal diabetes, and birth asphyxia, and is often self-limiting. Late hypocalcaemia (day 5 to 14) points to high-phosphate formula, maternal vitamin D deficiency, hypomagnesaemia, or DiGeorge syndrome. A neonate with late hypocalcaemia, a cardiac murmur, and an absent thymic shadow has 22q11.2 deletion until proven otherwise. Symptomatic neonatal hypocalcaemia is treated with the same slow intravenous calcium gluconate. [8]

Examiner traps

• Giving intravenous calcium as a rapid push (bradycardia and cardiac arrest). [1]
• Not checking magnesium in refractory hypocalcaemia. [3]
• Using native vitamin D instead of calcitriol in hypoparathyroidism (it cannot be activated without PTH). [1]
• Targeting a high-normal calcium and causing hypercalciuria and nephrocalcinosis. [7]
• Labeling high-PTH hypocalcaemia as hypoparathyroidism (it is pseudohypoparathyroidism or vitamin D deficiency). [4]
• Missing DiGeorge in a neonate with hypocalcaemia and a heart murmur. [8]