Calcium Homeostasis

Key points

• • Plasma calcium is roughly half ionised and physiologically active, about 40 per cent protein-bound and the remainder complexed, so a low albumin lowers total calcium without lowering the ionised fraction.
• • Alkalosis increases calcium binding to albumin, so the ionised fraction falls and tetany may appear even when total calcium reads normal.
• • Parathyroid hormone from chief cells raises calcium by increasing bone resorption, distal tubular calcium reabsorption and renal 1-alpha-hydroxylase activity, while increasing phosphate excretion.
• • Calcitriol, the 1,25-dihydroxy form of vitamin D, raises both calcium and phosphate mainly by increasing intestinal absorption.
• • Calcitonin from thyroid parafollicular C cells opposes parathyroid hormone but has only a minor role in human calcium balance.
• • Hypocalcaemia shows as neuromuscular irritability with the Trousseau and Chvostek signs; hypercalcaemia shows as lethargy, constipation and polyuria.