We have described this already in Chap. 1. Briefly, serum potassium is kept in the range 3.5–5.4 mmol/l by a number of important mechanisms. Normal dietary K intake is 50–100 mmol per day. Ninety percent of body’s 3,500 mmol K is intracellular and <1 % intravascular. Na/K pumps on cell membranes pump K into cells in exchange for Na under influence of insulin, beta agonists, alkalosis and aldosterone. Ninety percent excretion of K occurs through kidneys and only 10 % through colon unless patient has diarrhoea. Nearly all filtered K is reabsorbed and nearly all urine K is secreted by cells in the cortical collecting duct: aldosterone and alkalosis both lead to loss of K by these cells

Common things occur commonly. Diuretics, nutritional causes including refeeding syndrome, vomiting, diarrhoea and acute illness were the five most common causes of hypokalaemia in a consecutive series of 61 patients with serum K <2.5 mmol/l seen in south west Scotland in 2010 (Fig. 6.1)

Artefact accounted for two of the patients in this series: one who had blood taken from a drip arm and another during a massive blood transfusion. One patient had renal tubular acidosis and one patient with hypertension and hypokalaemia probably had primary hyperaldosteronism but declined further investigations. There were no cases of Bartter’s syndrome, Gitelman’s syndrome, Liddle’s syndrome or hypokalaemic periodic paralysis in this study.

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