Imagine that there are 100 patients who have hypothyroidism. Some of these may have Hashimoto's thyroiditis and some may not.
Imagine that out of the 100 patients about 10 of these appear to do sort of OK on synthetic T4. At least they feel well enough to not want to bother looking for another solution. The other 90 patients still felt ill enough that they wanted find a better solution than T4 (synthroid or levothyroxine).