I was going to suggest sodium as pizza and cheese are loaded with it. Perhaps increase your dose despite what your labs say, see if brain fog improves?
Other thing that comes to mind is candida - the lactose and carbs from cheese and pizza would give it some food and keep it happy, which i found helped improve brain fog when i ate food candida LOVES.
So maybe acetylaldehyde, the bi-product of candida breaking-down is causing your brain fog? It's the same stuff that swirls around the brain giving a hang-over after alcohol consumption.
When you eat candida friendly food it grows and stops breaking down, clearing acetylaldehyde from your system giving you more clarity, perhaps?
Are you generally on a diet which starves candida?