Hypophosphataemia is a lower-than-normal concentration of phosphate in the blood — below about 0.8 mmol/L. Phosphate is the backbone of ATP, cell membranes, and bone mineral, so severe deficiency starves every energy-hungry tissue: muscle weakness, respiratory failure, confusion, and the red-cell fragility of haemolysis. It strikes classically in refeeding syndrome — when starved patients are refed and insulin drives phosphate into cells — and with alcoholism, diabetic ketoacidosis treatment, and phosphate-binding antacids. Mild cases need only dietary phosphate (dairy, meat, nuts); severe, symptomatic cases need careful intravenous replacement. Like many electrolytes, phosphate matters most when it crashes — the refeeding ward’s most watched number.