There are mental illnesses, such as schizophrenia, that affect a person’s intelligence (decrease it.) Bipolar d/o is definitely not one of them. Bipolar d/o is strictly a mood disorder, meaning that one’s mood is affected. You can be severely depressed or severely manic, or a mixture of the two. Your IQ does not decrease, now or in time as a result of having bipolar d/o.
For example, even when I am very sick, I can still have an intelligent conversation and cite references as to the evidence of what I am saying. My mood might be awful, I may be weepy, hopeless, bone weary and irritable or I might be talking a mile a minute and very energetic, but through it all, I can carry on an intelligent conversation.
One caveat to this is when someone is out of touch with reality, or psychotic, which is the medical term, then you are not going to have a very intelligent conversation with them. They might tell you about a witch who is trying to damage their heart and how they have to burn basil to get her and her negative energy out of their house. That really happened to me, I truly thought there was a witch practicing black arts, who was after me and my heart. I literally felt chest pain and doubled over in pain, because I believed the witch had hurled damaging energy at me from somewhere in Europe. Pretty amazing what the mind can do. I would believe all that and then I would come out of it and be amazed at what I had been thinking. Anyway, if someone is in a psychotic phase, you can’t have an intelligent conversation with them unless you want to talk about witches who practice the black arts! However their intelligence is not affected by this and when they come out of this phase, they are normal again and you can converse with them normally.