Because the pituitary produces TSH does not necessarily mean that it gets into the thyroid gland to signal that more thyroxine is needed. The "door" to the thyroid must be opened with two keys. These are Vitamins C and B2. If they are low or missing, TSH will not be effective in signalling the thyroid to make thyroxine and problems will follow. When the TSH gets into the thyroid it requires Iodine to make thyroxine. No iodine - no thyroxine!! The takeaway message from this section is that for proper thyroid function, Vitamins C and B2 are needed plus dietary iodine. If this functions correctly thyroxine is released into the bloodstream. Unfortunately this is an inactive form (T4). Why evolution has made it so I cannot speculate - but this is how it is. The inactive T4 proceeds via the bloodstream to the liver and the next stage of the process. See post 3.
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