Visual summary
Confirm ionized calcium when needed, then ask whether PTH is an appropriate response before choosing a long-term replacement strategy.
Text version
Check the biologically active fraction
Ionized calcium is the active fraction. Total calcium falls with low albumin even when ionized calcium is normal; alkalemia lowers ionized calcium by increasing binding. In critical illness or discordant symptoms, measure ionized calcium rather than relying on an albumin-correction formula.
Recognize the urgent syndrome
Tetany, seizures, laryngospasm, significant QT prolongation, or arrhythmia with hypocalcemia requires urgent monitored assessment and often IV calcium. The speed of decline matters. Do not wait for a rigid total-calcium threshold when convincing severe symptoms are present.
Read PTH relative to calcium
Low calcium should stimulate PTH. Low or “normal” PTH may therefore be inappropriate and suggests hypoparathyroidism, magnesium deficiency, or related suppression. Elevated PTH shifts attention to vitamin D deficiency, impaired kidney activation, resistance, or acute binding/redistribution.
Use phosphate and magnesium
High phosphate with low/inappropriate PTH fits hypoparathyroidism; low phosphate with raised PTH fits vitamin D deficiency in an appropriate setting. CKD changes these patterns. Severe magnesium deficiency impairs PTH secretion/action and can make calcium difficult to correct.
Treat cause and route together
Stable chronic hypocalcemia can often use oral calcium and a cause-specific vitamin D strategy. Hypoparathyroidism or advanced CKD may require active vitamin D under specialist care. Check surgery history, denosumab/bisphosphonates, transfusions, malabsorption, and renal function.
Follow the treatment effect
Monitor symptoms, ECG when indicated, calcium, magnesium, and phosphate during acute therapy. In chronic treatment, avoid hypercalcemia and excessive urinary calcium. TLS-related asymptomatic hypocalcemia has a different precipitation risk; it is not a routine calcium-replacement indication.