Severe muscle weakness due to hypokalemia as a manifestation of small-cell carcinoma.

We describe the case of a 56-year-old man with severe muscle weakness due to heavy hypokalemia (serum potassium 1.44 mmol/l) associated with inappropriate kaliuria and alkalemia. Subsequent investigation revealed the presence of ectopic ACTH hypersecretion due to a small-cell lung carcinoma. A significant clinical/laboratory improvement was achieved following chemotherapy-induced regression of the primary tumor. The profound hypokalemia was probably the result of cortisol hypersecretion, which in concert with the ACTH-induced decreased 11beta-hydroxysteroid dehydrogenase activity can exhibit an increased mineralocorticoid activity. In addition, other ACTH-dependent mineralocorticoids may play a contributory role in the development of severe hypokalemia.