NDT is "natural dessicated thyroid".
The first successful treatment for hypothyroidism was giving people bits of raw sheep thyroid to eat. It was discovered very quickly that (surprise, surprise!) people didn't like taking it very much, but luckily it could be dried (dessicated) and made into pills and it still worked. This all happened near the end of the 19th century, in the 1890s. The pills created were, effectively, the treatment patients now call NDT.
NDT was the only treatment or the main treatment for hypothyroidism up until (roughly) the 1970s. Then pharmaceutical companies developed levothyroxine, and doctors decided that, since it was new and not natural (and therefore must be better), they would prescribe it for their patients and stop prescribing NDT. Many of their patients hated levo but the doctors knew better than their patients what was good for them so they had to carry on with levo.
These days many doctors have never even heard of NDT. Those that have think it is the kind of stuff that witchdoctors produce. And yet plenty of people buy NDT over the internet and live happily ever after. Unfortunately doctors and pharmaceutical companies don't like people getting better without spending money on doctors and pharmaceutical drugs so one day, probably, NDT will be banned. The fact that the people who will suffer are mainly women means it isn't very important, and doctors and pharmaceutical companies don't need to worry about them. Everyone knows their symptoms are all in their heads after all, so they don't count.
(My apologies - I tend to get angry and sarcastic when I think about pharmaceutical companies and doctors.)