In the most basic terms, what is the difference ? Why could it be that some people are more comfortable with NDT while the Levo did not work for them ? Are there any people who have experiences the other way also i.e. NDT did not suit them while Levo did ?
If I want to try NDT, Can I buy it off the shelf in the UK (Assuming GP won't prescribe) ?
If not why not ?
-Sat
Levothyroxine is wholly synthetic and is made in a factory.
NDT is natural dessicated thyroid. It starts off life being the thyroid of a pig. Lots of pig thyroids are pulverised and dried. Then fillers are added, and the dessiccated thyroid gets made into pills. A pharmaceutical chemist would probably laugh his socks off at my description, but its how I always imagine it happens.
You can go to a supermarket and buy pork chops, pig liver, pig heart, pork sausages etc, but buying pig thyroid pills is not possible. It is classified as a prescription-only medicine, just like levothyroxine.
Levothyroxine contains only one thyroid hormone, referred to as T4.
NDT contains all the thyroid hormones that a mammal thyroid makes - T4, T3, T2, T1 and calcitonin, and any hormones or other substances that have yet to be discovered. The proportions of the different thyroid hormones in the thyroid differs between species, and doctors often use this fact to claim that pig thyroid is very bad for us so we shouldn't take NDT. Instead we should take 100% T4, 0% T3, 0% T2, 0% T1 and 0% calcitonin plus 0% of all the unknown hormones they don't yet know about in levothyroxine pills.
Sat77, as Humanbean explained, NDT is prescription only in the UK and that's why it is not available over the counter. It isn't licensed for UK use but doctors can prescribe it on a named patient basis if their local CCG permits. Most won't because they aren't trained to use it, don't understand what it is, and are unwilling to jeopardise their position by prescribing outside of BTA guidelines which oppose it's use.
thyroiduk.org.uk/tuk/treatm...
Some people don't convert T4 to sufficient T3 on Levothyroxine and need the addition of T3 either synthetic or via NDT. Some don't tolerate synthetic meds and do better on NDT. There are members who've not done well on NDT and have switched back to Levothyroxine.
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Disclaimer: I am not medically trained. My advice is based on my personal experience &/or research and is not given in my capacity as an Admin of Thyroid UK or endorsed by TUK. Information on the forum is not intended to be a substitute for medical guidance from your own doctor.
Sounds a bit frustrating - On one hand it is an approved prescription medication (named patient basis or whatever), On the other hand it could be unsafe !! On one hand the GP would not prescribe it easily ( Isn't there a cost angle as well there ? ) and On the other hand we can't buy it ourselves.
If someone is in discomfort, the GP doc has nothing to lose and they do not want to do anything that could deviate from norms. Whose quality of life is at stake, anyhow !!
If you live in the UK it is legal to buy (from abroad) and import prescription-only medicines (including NDT) for your own use or for the use of a member of your household.* There are some countries where NDT is made and can be sold legally without a prescription. Put the two together and it means you can buy NDT online from some sources. If your package is stopped by customs you will probably have to pay VAT, customs duty and admin charges.
GPs are not happy about prescribing any drug on a Named Patient Basis because it means that they are taking personal responsibility for your health, and they can be sued personally if it goes wrong. If they stick to prescribing only those drugs that are given in guidelines, and they stick to guidelines in treating you then they are exonerated from blame and cannot be held personally responsible.
* This does not apply to controlled drugs
Levo is T4 alone most people have a problem converting T4 to T3.
To answer another part of your question, no, NDT does not suit everyone. Especially if you have Hashi's, it can exacerbate antibody attacks. Also, there are some people - like me - who cannot tolerate any form of T4 and do better on T3 alone, even though it is synthetic. We are all different, and it's a matter of trial and error to find what suits us best. 
Switching from ndt to Levo 
prescription NDT vs Thai NDT
Changing from Levo to NDT
Increase in Levo or move to NDT
