Tocilizumab and electrolyte imbalance: severe hypokalaemia leading to paralysis

Tocilizumab, an interleukin-6 (IL-6) receptor antagonist, is widely used for inflammatory diseases such as rheumatoid arthritis and adult-onset Still's disease (AOSD). While its known side effects include hypersensitivity, neutropenia and liver dysfunction, hypokalaemia is a rare adverse effect. We report a male patient in his early 30s with AOSD who developed severe hypokalaemia (2.0 mmol/L) and acute flaccid paralysis after his third intravenous tocilizumab infusion. He had no prior neurological or electrolyte disturbances. Electrocardiography showed U waves and mild QT prolongation, while brain and spinal MRI ruled out other causes. Potassium replacement led to full recovery within 24 hours, and his treatment was switched to adalimumab. This case suggests that IL-6 inhibition may disrupt potassium homeostasis, leading to hypokalaemia. Hypokalaemia should be kept in mind in cases of sudden onset of paralysis under tocilizumab treatment.