I have been known to get this brain fog also. For me it seems to be a combination of depression and a stress overload. My old therapist once compared it to an electronic overloading, she said your brain can only handle so much stress before you go on autopilot.

Having your thyroid checked is always a good idea however there are also other tests that can be done to eliminate other possibilities. One time my electrolytes were out of balance and I felt completely out of whack.

I hope things improve for you soon, the brain fog is no fun at all.