ask GP to check levels of vitamin d, b12, folate and ferratin. These all need to at good (not just average) levels for thyroid hormones (our own or replacement ones) to work in our cells.
Common to be low in any or all of these, especially if under treated. Low b12 causes brain fog and confusion. Any result under 500 on B12 is suspious, especially if also low in folate.
Also have you had thyroid antibodies checked? There are two sorts TPO Ab and TG Ab. (Thyroid peroxidase and thyroglobulin) Both need checking, if either, or both are high this means autoimmune thyroid - called Hashimoto's, the most common uk cause of being hypo. NHS rarely checks TPO and almost never checks TG. NHS believes it is impossible to have negative TPO and raised TG. It's rare, but not impossible.
Make sure you get the actual figures from tests (including ranges - figures in brackets). You are entitled to copies of your own results. Some surgeries make nominal charge for printing out.
Alternatively you can now ask for online access to your own medical records. Though not all surgeries can do this yet, or may not have blood test results available yet online.
When you get results suggest you make a new post on here and members can offer advice on any vitamin supplements needed
If you can not get GP to do these tests, then like many of us, you can get them done privately
Blue Horizon - Thyroid plus eleven tests all these.
Best advice is to read as much as you can. Vitamin and minerals levels are very important, but standard NHS thinking, doesn't at the moment seem to recognise this.
Chicken & egg - low thyroid= low nutrients & low nutrients often means you can't uptake thyroid hormones.
You will see, time and time again on here lots of information and advice about importance of good levels of B12, folate, ferritin and vitamin D and gluten connection to autoimmune Hashimoto's too.
Leaky gut and/or low stomach acid implicated too
See - Thyroid uk.org