Chapter 6: Calcium, Phosphate and Magnesium

Calcium, Phosphate and Magnesium explores the homeostatic regulation of these electrolytes through the synergistic actions of parathyroid hormone (PTH), calcitriol (1,25-dihydroxyvitamin D), and calcitonin, which modulate intestinal absorption, renal excretion, and bone turnover. A significant portion is dedicated to the clinical assessment of plasma calcium, emphasizing the necessity of calculating adjusted calcium levels to account for protein-binding fluctuations, primarily involving albumin. The text provides an in-depth analysis of hypercalcaemia, often linked to primary hyperparathyroidism or humoral hypercalcaemia of malignancy, and its symptomatic presentation commonly remembered as "bones, stones, abdominal groans, and psychic moans." Conversely, it details the pathophysiology of hypocalcaemia, resulting from conditions like vitamin D deficiency, renal osteodystrophy, or hypoparathyroidism, leading to neuromuscular irritability such as tetany. Furthermore, the chapter reviews phosphate's critical function as an intracellular buffer and its involvement in metabolic pathways like glycolysis and oxidative phosphorylation, alongside magnesium’s role as an essential enzyme cofactor. Diagnostic methodologies, including the interpretation of alkaline phosphatase activity and the use of the calcium-sensing receptor (CaSR) in metabolic control, are thoroughly discussed. Finally, it differentiates between primary bone pathologies like osteoporosis and Paget’s disease, providing a comprehensive framework for differential diagnosis and therapeutic management in clinical chemistry.