Chvostek's sign and Trousseau's sign are the two classic clinical tests for neuromuscular excitability from hypocalcaemia. Chvostek's sign: tap the facial nerve anterior to the ear — ipsilateral facial muscle twitching = positive (sensitivity 70%, specificity 90% for hypocalcaemia). Trousseau's sign: inflate BP cuff above systolic for 3 minutes — carpopedal spasm (wrist flexion, thumb adduction, finger extension) = positive (sensitivity 94%, specificity 99% for symptomatic hypocalcaemia). Trousseau's sign is more sensitive and should be checked in all cases of suspected hypocalcaemia where the patient has perioral tingling, paraesthesia, or muscle cramps. A positive Trousseau's with a calcium of 1.9–2.1 mmol/L indicates the patient is clinically symptomatic and requires urgent treatment even though the biochemical level appears moderate. The QT prolongation mechanism of hypocalcaemia (elongation of the intracellular action potential plateau by reducing calcium-mediated channel inactivation) is important to recognise — the classic ECG finding is a prolonged ST segment (not a prolonged T wave as in other QT-prolonging conditions). Coexisting hypokalaemia and hypomagnesaemia (common in critically ill patients) dramatically increase the arrhythmia risk from hypocalcaemia-related QT prolongation. These electrolytes must be corrected simultaneously.

The corrected calcium calculation is one of the most commonly performed and most commonly wrongly interpreted calculations in primary care. Laboratories differ in whether they auto-correct calcium for albumin — some report only total calcium with a separate albumin result, requiring the GP to calculate the corrected value manually. Always check whether the calcium result reported is the corrected value. Ionised calcium (directly measured by blood gas analyser) is more accurate than calculated corrected calcium, particularly in acid-base disturbances (alkalosis reduces ionised calcium without changing total calcium — hyperventilation causes acute hypocalcaemia by respiratory alkalosis, which is why anxious hyperventilating patients develop perioral tingling and carpopedal spasm). In a patient with an arterial blood gas available, the ionised calcium is the most reliable measure. The important clinical principle in a well, asymptomatic patient is: do not prescribe calcium supplements based on a laboratory total calcium result without calculating the corrected value first — you may be treating a patient with normal ionised calcium based on a spuriously low total calcium from hypoalbuminaemia, unnecessarily exposing them to hypercalcaemia risk.

Denosumab-induced hypocalcaemia is an increasingly important clinical problem — as denosumab use for osteoporosis has expanded, the risk of severe symptomatic hypocalcaemia (requiring hospitalisation) has become clinically significant. Denosumab (Prolia 60 mg SC 6-monthly for osteoporosis, or Xgeva 120 mg monthly for bone metastases) powerfully inhibits osteoclast-mediated bone resorption, removing the main source of calcium efflux from bone. Patients with pre-existing vitamin D deficiency or CKD are at highest risk. Before and during denosumab: (1) correct pre-existing hypocalcaemia and vitamin D deficiency before starting (MHRA Drug Safety Update, September 2014; in practice aim for 25-OH vitamin D >50 nmol/L), (2) ensure adequate calcium and vitamin D intake or supplementation throughout (MHRA Drug Safety Update, September 2014), and (3) monitor calcium in those predisposed to hypocalcaemia, especially eGFR <30 (MHRA; product information: calcium before each dose, plus a check within 2 weeks of the first dose in those predisposed, e.g. CrCl below 30). GPs prescribing denosumab must understand this risk and implement these precautions. Vitamin D deficiency is by far the most common cause of hypocalcaemia worldwide — the WHO estimates 1 billion people have deficient vitamin D levels. In the UK, the highest risk groups are: South Asian and Black ethnicities (reduced cutaneous vitamin D synthesis due to melanin pigmentation), housebound and elderly individuals, pregnant and breastfeeding women, patients with malabsorption syndromes, and patients on anticonvulsants or rifampicin (hepatic induction of vitamin D catabolism).

Magnesium depletion as a cause of hypocalcaemia is frequently missed and is the most important reversible cause of treatment-resistant hypocalcaemia. Magnesium is required for PTH secretion — when magnesium is depleted (<0.6 mmol/L), the parathyroid glands cannot secrete PTH in response to low calcium, producing a functional hypoparathyroidism. Additionally, even if PTH is secreted, hypomagnesaemia causes PTH resistance at the bone and kidney. The critical clinical principle: if a patient with hypocalcaemia is not responding to calcium and vitamin D supplementation, measure magnesium — and if low, replace magnesium first (IV magnesium sulfate in hospital if severely depleted). Common causes of magnesium depletion: alcoholism (most common), proton pump inhibitors (chronic use depletes intestinal Mg absorption — PPIs specifically block the TRPM6 channel responsible for intestinal Mg absorption), cisplatin chemotherapy, loop diuretics, diarrhoea. The PTH result is the key diagnostic branch point in hypocalcaemia investigation — a low or inappropriately normal PTH in the context of hypocalcaemia indicates hypoparathyroidism (the parathyroids are not responding normally to the low calcium signal); a high PTH indicates secondary hyperparathyroidism (the parathyroids are appropriately responding but cannot maintain normocalcaemia due to vitamin D deficiency, CKD, or malabsorption). This single result directs the entire subsequent workup and treatment pathway.

The management of hypoparathyroidism (low PTH hypocalcaemia) is fundamentally different from vitamin D deficiency hypocalcaemia and should not be managed in primary care without specialist input. The treatment of hypoparathyroidism uses calcitriol (active vitamin D, 1,25-dihydroxycholecalciferol — bypasses the renal 1-hydroxylation step that requires PTH) rather than cholecalciferol (inactive, requires PTH-stimulated renal 1-hydroxylation to become active). Prescribing cholecalciferol alone for hypoparathyroidism is ineffective and potentially dangerous — it gives false reassurance that the vitamin D deficiency is being treated while the patient remains hypocalcaemic. Calcitriol dosing requires specialist monitoring because the therapeutic window is narrow — too little = persistent hypocalcaemia; too much = hypercalcaemia with hypercalciuria → nephrocalcinosis → irreversible renal damage. Recombinant PTH (teriparatide, or PTH 1-84 — rhPTH) has been used for hypoparathyroidism (Natpar, now discontinued by its manufacturer; palopegteriparatide is the newer option) and is initiated by endocrinologists for difficult-to-control hypoparathyroidism, particularly when calcitriol requirements are very high or hypercalciuria is problematic.

The choice between calcium carbonate and calcium citrate for oral calcium supplementation has an important practical implication — calcium carbonate requires gastric acid for dissolution and absorption, so it must be taken with food (the meal stimulates gastric acid secretion). Patients on proton pump inhibitors (PPIs) have reduced gastric acid and absorb calcium carbonate poorly — studies show 50% reduced calcium absorption in PPI-treated patients compared to those with normal gastric acidity. For patients on PPIs or with achlorhydria (e.g., post-gastrectomy), calcium citrate is the preferred formulation as it does not require acid for absorption. The practical GP instruction: "Take your Adcal/Calcichew WITH your main meal, not fasting." Many patients take their calcium supplement in the morning with water and a PPI — absorbing very little. The colecalciferol loading dose approach (300,000 IU over 6–10 weeks) is the Royal Osteoporosis Society (Vitamin D and bone health) strategy for treating vitamin D deficiency — it rapidly restores 25-OH vitamin D levels within 6–8 weeks versus the 3–6 months required with maintenance dosing alone. Loading should follow local vitamin D guidance and patient-specific contraindications (renal impairment, granulomatous disease, primary hyperparathyroidism, malabsorption, stones) — it is not a universal treatment for hypocalcaemia. The exact schedule varies by formulation: 50,000 IU (colecalciferol) weekly × 6 weeks is a convenient approach using standard UK preparations.

The target calcium for hypoparathyroidism is deliberately set at low-normal (2.0–2.25 mmol/L) rather than mid-normal — this reflects the paradox of treating hypoparathyroidism with calcitriol and calcium: without PTH, there is no mechanism to reduce urinary calcium excretion when serum calcium rises (PTH normally increases tubular calcium reabsorption). High-normal calcium in the absence of PTH leads to hypercalciuria even when serum calcium is within the normal range, causing progressive nephrocalcinosis and renal failure. Long-term nephrocalcinosis is the most serious chronic complication of hypoparathyroidism and is directly caused by overtreatment. The sick day rules for hypoparathyroidism are one of the most important patient education elements — unlike adrenal insufficiency (which can be treated with stress-dose steroids) or diabetes (with sliding scale insulin), hypocalcaemic crises in hypoparathyroidism are directly precipitated by missed calcium and calcitriol doses during illness. A hypoparathyroid patient who vomits for 24 hours and cannot take their calcitriol and calcium has no parathyroid gland backup and will develop symptomatic hypocalcaemia rapidly. These patients should have a written specialist emergency plan: if oral calcium/calcitriol cannot be retained or absorbed, contact the endocrine team or attend hospital urgently — no unsupervised calcitriol increases.

The "calcium and bisphosphonate timing" principle is critically important for prescribing practice — any divalent cation (calcium, magnesium, iron, zinc, aluminium-containing antacids) chelates bisphosphonates in the gastrointestinal tract, preventing their absorption. Bisphosphonates (alendronate, risedronate, ibandronate) should be taken on an empty stomach, 30 minutes before breakfast, with a full glass of plain water, without any other medication or food. This is frequently not explained to patients, and taking the bisphosphonate with food, other medications or the calcium supplement prescribed alongside greatly reduces its absorption. The MHRA guidance is explicit: take bisphosphonate first thing in the morning, wait 30 minutes, then take all other medications including calcium. A practical patient instruction card is available from the Royal Osteoporosis Society. The denosumab pre-treatment vitamin D optimisation is one of the most frequently neglected prescribing steps — the MHRA received multiple yellow card reports of severe hypocalcaemia following denosumab initiation in vitamin D-deficient patients. Checking calcium, renal function and vitamin D before starting denosumab, and correcting deficiency, is essential safe prescribing; repeat checks follow individual risk and the current local shared-care protocol.

Patients recently discharged after thyroid or parathyroid surgery should follow the operating team's monitoring and supplementation plan; the GP's role is to recognise new symptoms and arrange urgent assessment. Total thyroidectomy (removal of entire thyroid gland) can cause permanent hypoparathyroidism from inadvertent removal or devascularisation of the parathyroid glands, and temporary hypocalcaemia is common in the first 24–72 hours. Patients are discharged with instructions for calcium monitoring and should contact their GP or the surgical unit if they develop any perioral or fingertip tingling. The GP receiving such a patient should check corrected calcium and PTH urgently — perioral tingling within 72 hours of thyroidectomy is treated as calcium deficiency until proven otherwise, even if the patient looks well. Corrected calcium <2.0 in this context → contact the surgical team immediately and follow their supplementation plan. Bone density in hypoparathyroidism is assessed only as the specialist directs, and is read differently from most patients — hypoparathyroidism (the absence of PTH-driven bone resorption) causes increases in bone mineral density (BMD), particularly at cortical sites. While this might seem beneficial, it can mask the trabecular bone changes that predict fracture risk in other situations, and the high BMD can generate false reassurance about fracture risk.