What we know (or think we know) is that TSH is a type of "running average" of our actual thyroid hormone levels. Thyroid hormone levels — especially the T3 — can be up and down more quickly than the TSH moves. So, if you want, think of it as T4 and T3 levels being a "snapshot," while the TSH is a "timed exposure." It usually lags a bit behind our normalizing thyroid levels. Conversely, it can lag a bit behind if the thyroid levels are going "off" again.
More importantly, though, it takes some time AT normal levels of thyroid hormone for our symptoms of hyper to go away completely. I think of it in terms of the body needing time to heal from whatever damage is done. Regardless, there is no magic "get to normal levels and feel instantly and completely normal" experience. We gradually return to our old selves after weeks or months AT normal levels of thyroid.
Also, stress causes the release into the body of other chemicals from the endocrine system….especially the adrenals, I believe. So the hormone soup running through your veins changes with stress, changing the internal balance again.
