Wikipedia reports TSH is generated by the pituitary gland, and that's right next to the pineal gland/ circadian rhythm regulator.. Both are very sensitive to fluoride levels.
Perhaps the pituitary is requesting an upped metabolism when you're on lower Iodine doses because it's compensating for the downregulation effect from fluoride exposure?
Iodine and boron both displace the fluoride halogen.
What's your fluoride intake look like? Aka, levels in tap/drinking/bathing water, toothpaste, teas and coffee, soil levels where your fav foods are grown...
Maybe there's a clue?