Hypo can cause brain fog, but so can other things. You might want to have vitamin B-12 tested, as well.
Please post your thyroid levels, when you get the results and be sure to include reference ranges, which vary lab to lab and have to come from your own report.
I sure hope that is true. I am feeling like an idiot way too often!
Yes, slowing down of mental function is a symptom of hypothyroidism. I had this badly. There is an old medical saying: "just a few grains of thyroid hormone can make the difference between an idiot and an Einstein." :)