Hi there, thank you for posting your question and for your excellent descriptions.
I will make a few bullet points first:
1- An adenoma does not usually require long term suppression of TSH, if it is benign. Only in some cases of thyroid cancers that suppression of TSH is indicated
2- Even then, there is much debate about the need for aggressive suppression, as some recent studies have shown no benefits, and only moderate suppression of TSH is enough.
3- Desiccated thyroid is a mixture of thyroid hormones, T4 and T3, with no strict quality control of how much of each is in each pill. So for long term treatment things balance out, especially in patients who are trying to "take too much" by definition to over-suppress their TSH. But if you try to titrate the dose of desiccated thyroid to maintain a certain level of thyroid hormone and TSH in the blood it will become more difficult. Use of pure T4 levothyroxine is much better for that.
4- We must balance between the need for TSH suppression and the risks from hyperthyroidism. By definition, a low TSH means you are hyperthyroid and have been for 20 years, which may feel relatively good because you have a lot of energy and you may not have a problem with weight gain. But with palpitations (and long term effects on bones), you need to be deciding if it is really necessary to remain hyperthyroid.
5- After long term suppression of TSH, it will take a while to get the levels back up, even if your Free T4 and T3 go down, it may take several months before your TSH can increase. This is to put your most recent values in perspective (the TSH was still very low even though thyroid level were almost half what they were before that.
6- With increasing age, some people start requiring less thyroid hormones, so their dose needs to be adjusted accordingly.
So, with all that, my first recommendation is for you to see an endocrinologist who is up to date on the recommendations and guidelines, and find out if your adenoma was benign or malignant, (and what type of cancer was it), and whether the "adenoma" was removed or the whole thyroid gland was removed. He/she will probably switch you to T4 preparation and monitor your thyroid levels much better. With all that information, it may turn out that you don't really need suppression of TSH.
Long answer but I hope I provided with the information you need.