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 is: never 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. NICE guidance mandates: (1) correct vitamin D deficiency before starting denosumab (ensure 25-OH vitamin D >50 nmol/L), (2) prescribe calcium and vitamin D supplementation with every denosumab dose, and (3) check calcium 2 weeks after each injection in high-risk patients. 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) is now licensed for hypoparathyroidism in Europe (Natpar) 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 NICE-endorsed 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. A single-point high loading dose is more convenient and more effective for achieving rapid correction in symptomatic deficiency. 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 emergency plan and emergency stock of calcium tablets at home.

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 — compliance data show that 30–40% of patients take their bisphosphonate incorrectly (with food, with other medications, or with the calcium supplement prescribed alongside). This renders the bisphosphonate completely ineffective. 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 25-OH vitamin D before starting denosumab (or before EVERY 6-monthly Prolia injection in high-risk patients) and ensuring levels are >50 nmol/L is mandatory safe prescribing.

Post-thyroidectomy hypocalcaemia monitoring is one of the most important safety protocols in post-surgical GP care. Total thyroidectomy (removal of entire thyroid gland) carries a 1–2% risk of permanent hypoparathyroidism from inadvertent removal or devascularisation of all four parathyroid glands, and a 5–10% risk of temporary hypocalcaemia 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 surgical team immediately and initiate oral calcium (Adcal 1.5 g BD) while awaiting results. The annual DEXA in hypoparathyroidism is done for the opposite reason to 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.