You must have your FT3 and FT4 tested. They should be at least middle to upper half of the reference range.
In some people who have been on thyroid meds for a long time, the link between the FT3 and FT4 and TSH tends not to work well any more.
Your doctor should be medicating you on the basis of FT3 and FT4, not TSH. it is highly unlikely that your thyroid has started working again after 24 years of meds. So the other explanation is that the TSH is no longer a good indication of your thyroid hormone levels.
Sounds like you are a victim of the "Immaculate TSH Belief". Many doctors only want to go by TSH in dosing a thyroid patient. This doesn't really work. TSH is a pituitary hormone that is affected by so many variables that it does not even correlate with the biologically active thyroid hormones, free T3 and free T4 (not the same as total T3 and total T4), much correlate with symptoms. FT3 is the most important because it largely regulates metabolism and many other body functions. Studies have shown that FT3 correlates best with hypo symptoms, while FT4 and TSH did not correlate.
In my opinion the best way to treat a thyroid patient is to test and adjust FT3 and FT4 with whatever meds are required to relieve symptoms, without being constrained by resultant TSH levels. Symptom relief should be all important, not TSH level. I say this about TSH levels because many patients report having TSH suppressed when on thyroid meds. This does not mean you have become hyper, unless you also have hyper symptoms and also excessive levels of FT3 and FT4. Unfortunately many doctors do not understand this and immediately want to reduce your thyroid meds, making you even more hypo.
I think you are going to have to discuss this with your doctor and try and make hinm understand that you want to be treated clinically (for symptoms), by testing and adjusting FT3 and FT4, without being constrained by TSH. If the doctor won't treat you in this manner you will have to find a good thyroid doctor that will do so. For discussion with your doctor you might find a copy of this link to be useful.
http://www.hormonerestoration.com/Thyroid.html