Can having depression or bipolar disorder cause you to have a lower iq?

Probably not: Depression, or bipolar disorder, are illnesses. Like any other illness, they can affect your ability to think or to process information. Iq is a relatively fixed quantity. So, it is likely that if you are, for example, depressed, you may test at a lower iq. But, once you get better, you would test at a level more reflective of your actual ability.

No, depends: If you are in the depths of depression, or spinning in a manic episode when your IQ is tested, you won't score as well as you ordinarily would. That said, depression & bipolar patients tend to be brighter than average. Interesting question. Why do you ask?

Lower IQ: Absolutely. Cognitive functioning and intelligence testing can be negatively impacted by emotional disorders.