Brief Answer:
you need supplementation and investigation for the cause

Detailed Answer:
Hello,

it's obvious that you have a vitamin B12 deficiency. This kind of deficiency may cause problems to your blood and nervous system. The causes may include:
- vegan/vegetarian diet
- drugs like metformin, proton pump inhibitors, H2 blockers, etc
- chronic gastrointestinal diseases (like Crohn's disease, gastrointestinal surgery, etc)
- other causes of malabsorption (pernicious anemia, atrophic gastritis, etc)

Early detection and treatment are very important to avoid permanent neurological damage.

A complete blood count and peripheral smear may show signs of macrocytosis, leukopenia, anemia, thrombocytopenia and the characteristic microscopic findings of hypersegmented polymorphonuclear cells and megalocytes.

Since your value is very low, there is no need for confirmation and treatment should start right away! Levels of folic acid in the blood should be measured too, because low folic acid values may cause low B12 values (folic acid supplementation would be needed in this case).

Antiparietal cell antibody should be measured to determine whether pernicious anemia is the cause. Fasting gastrin values should be elevated because of gastric achlorydria. Intrinsic factor antibody is a very specific test for pernicious anemia but not a very sensitive one (only 50% test positive) and should be done too.

If pernicious anemia is detected then keep in mind for the future that there is an increased (but not high in absolute numbers) risk for gastric cancer, so an upper endoscopy would be helpful.

Regarding supplementation, there are two options: oral or parenteral (cyanocobalamin or hydroxycobalamin, either is OK). In cases of pernicious anemia absorption is likely to be severely impaired, so the parenteral route is preferred. In the other cases, unless the patient has severe symptoms (neurological damage and/or severe anemia), the oral route is worth a try because it's easier and can be equally effective.

Oral administration of 1000mcg per day of vitamin B12 (in either of the two forms I've mentioned before) should be enough. This is more than the average needs but a higher than dosage is needed for adequate absorption.

Parenteral administration requires daily or every other day injections (one per day) for the first couple of weeks then monthly life long replacement treatment ever after. During the first few days brisk blood cell production may cause hypokalemia, so it has to be checked 3-5 days after the initiation of treatment.

Uploading your test reports (a complete blood count for example) and mentioning your symptoms, would probably help for a more individualized response.

I hope you find my comments helpful!
You can contact me again, if you'd like any clarification or further information.

Kind Regards!

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Brief Answer:
autoimmune thyroiditis is often accompanied by B12 deficiency

Detailed Answer:
Since your CBC is normal, it seems that this deficiency is recent. This is good. The symptoms are not related to B12 though. There are other autoimmune causes that may be related as well (provided that you have pernicious anemia) like vitiligo, type 1 diabetes, adrenal insufficiency, etc

The risk you'll acquire one of the aforementioned conditions is low but it's higher than in the general population.

Regarding management nothing changes by the diagnosis of thyroiditis. You should do the tests I've mentioned and depending on the results take either oral or parenteral supplementation.

Kind Regards!

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