If your doctor prescribes T3 for you, you get it from the pharmacy just the way you get any other drugs.
I have the impression from posts here (I'm not in the UK so I have no firsthand experience) that NDT is difficult or even impossible to get in the UK because so few doctors will prescribe it. So synthetic T3 prescribed by your endo could be a good start. If you feel fine on T3 and T4, once you've tweaked your dosages to find the optimal maintenance dosage for you, then you would not have to worry about anything.
I also imagine that prescription T3 is covered by health insurance, whereas prescription NDT most likely is not.
If you prefer NDT, for whatever reason, and cannot get a doctor to prescribe it, you can always order it online. There are several Thai suppliers of Thai NDT (three different brands), and a few US based pharmacies selling NDT (Armour, WP, NP) and delivering internationally (much more expensive than Thai NDT, though).
As many are struggling they decide to switch to NDT, again very rarely is it prescribed on the NHS.
You can purchase NDT from supplies in Thailand or USA (Armour).
You will find if you put your most recent blood results and give more background information, members will supply you with a reputable supplier.
Your GP may have a fit if you tell him you are self medicating, some are supported by their GP others aren’t and end up giving them a horror story about self medicating.
If you do this correctly it will work, off course you will have to arrange private bloods yourself to check how your thyroid is working.
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