Yes it will take about 6 weeks (maybe less, maybe more) for your body to settle into a constant level related to that dosage you are on.
As far as the brain fog, I can't say for sure on that one (I still have it with my problems
), but it should go away once your thyroid levels are set properly - also consider getting your adrenals checked out and getting on a good vitamin/mineral regimen (research on this one). It says on the prescribing information for all the thyroid hormones that any adrenal balance should be checked and corrected BEFORE thyroid supplementation begins, yet most doctors are NEGLIGENT in that they do not do this.
Hope this helps some.