What food are you feeding your ig? What kind of lighting? Do you have UVB lighting? If so what kind of bulb is it and how old is it? Distance from uvb bulb to the reptile matters and they should not have any sort of plastic/glass (or tiny wire mesh) between bulb and iguana.
You've got a problem going on the blood charts will give a better idea. Metabolic bone disease is a very common problem and that can cause seizures.. I believe renal failure can.. But I'm not in your shoes and have no idea what your day to day care is so I can list possibilities all day.