A patient is brought to the casualty in the state of altered sensorium. He was on Lithium treatment for affective disorder and has suffered through an attack of epileptic fits. On examination he has worsening 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 meg/Lit. The probable cause for his present state is: