You say you have had thyroid tests and they are "normal". Do you have actual results? (When you get hold of them - do a new post on here and members can advise)
Have you been tested for thyroid antibodies? There are two sorts TPO and TG. Either or both can be high, indicating autoimmune thyroid, called Hashimoto's. NHS rarely tests for TPO and almost never for TG. Hashimoto's often linked to gluten intolerance.
Many of us have had to resort to private blood tests to work out what is going on
Thyroid plus eleven from blue horizon includes vitamin D
Question is why are your vitamin D and B12 low? It could be gluten intolerance or coeliac. Have you had coeliac test? Can get false negative, and there is no reliable gluten Intolerance test. Only real way is to try adopting gluten free diet.
(I am not a medic, but have hashi and speak from personal experience - see my profile)
Norman centre in Florida - website is very good - also have made two apps.
One for vitamin D - called vitamin d pro - not free - I think about £1.50 to download to phone or tablet. Very good at explaining how low vitamin d can cause secondary Hyperparathyroidism. You can add your own blood test data, it works out likely cause/issues and does good graphs too
Similar one made called Calcium pro, mainly aimed at primary Hyperparathyroidism