It must be my turn to have a bad day.
The problem in a nutshell is this:
Hypothroidism (the only reason that one would take any of this stuff) requires medication. There is no other way to manage the disease. Contrary to what is out on websites, I and others have seen more trouble regulating metabolism with it than L-thyroxine. Part of that is that the amount of hormone needed is minute, with some people responding adequately to as little as tens of micrograms. Typically, we are talking 100 mcg or less for treatment. Many of the sites refer to dessicated thyroid as being superior because it's "natural". Well that means nothing of itself. You can find some foxglove, make an extract and drink it. If you have CHF, or "dropsy" as it was once known, you can certainly improve the condition. It's certainly cheap. The problem is that it can kill you. Why? Not because of anything in it, but plants (and thryoid) contain different amounts of active ingredient not directly related to weight. In many cases this would make little difference, but there is something known as the therapeutic index. Like digitalis and the other cardiac glycosides, thyroid has a narrow index, which means there isn't a great deal of difference between the amount that helps and the amount that kills. This is serious stuff, and while deaths are indeed rare, there is variation from lot to lot which can cause metabolism to be high sometimes and low the next. The feedback mechanism which controls this is highly sensitive, but that doesn't exist because someone is chomping down pills rather than having it released as appropriate. It's the very devil to get right, and with thyroid it's like playing whack a mole.
Now there are problems with treating thryoid, and some of the sites refer to them. One of the more serious is that we measure the active metabolites and determine how much to dose based on that. For the majority of patients, this works, but there is a significant population which doesn't correlate. That creates problems because to get relief of symptoms one sometimes has to increase to the extreme upper end of dosing, and it doesn't take much more to kill a person.
One thought is that there may be a receptor problem which makes a person resistant to thyroid hormones in a way that's analogous to type II diabetes, and no one is particularly pleased with this medical reality. That isn't something that thyroid "cures". It's a real problem and much needs to be done, however that's unrelated to L-thyroxine in and of itself.
Another thing is that a claim is made that this is an attempt to force people to take a more expensive patented medication, when in fact thyroid is more expensive and has been off patent for many years.
So there is a "conspiracy" to take a "less expensive" "harmless" drug off the market in order to force people to take a "patented" "dangerous" "more expensive" one.
It doesn't hold up.
Besides you are forgetting that the FDA has some latitude in what it does, but Congress is the driving force here. There is something known as DESI (look it up) which has been a pain in the neck to deal with. If anything the FDA has been dragging it's heels on a lot of things, and yes I do agree that they haven't everything right.
That said, it's best to be critical of things which are rather than inventing a crisis and sticking it to them based on that.
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