A pt. is brought to the casualty in the state of altered sensorium. He was on lithium t/t for affective disorder and has suffered through an attack of epileptic fits. On examination he has tremors increased DTR's and incontinence of urine. He has also undergone an episode of severe gastroenteritis 2 days ago. The serum lithium was found to be 1.95 m moles/L. The cause of her condition is :