Hypothyroidism slows metabolism and this can cause the parietal cells to produce less stomach acid (hypochlorhydria). Adequate levels of stomach acid is required for vitamin B12 absorption.
Another cause for vitamin B12 deficiency is due to other autoimmune disease that may occur with Hashimoto's thyroiditis such as autoimmune pernicious anaemia or celiac disease.
Prevalence and relative risk of other autoimmune diseases in subjects with autoimmune thyroid disease. Am J Med. 2010; 123(2):183.e1-9 (ISSN: 1555-7162)...
"The frequency of another autoimmune disorder was 9.67% in Graves' disease and 14.3% in Hashimoto's thyroiditis index cases (P=.005). Rheumatoid arthritis was the most common coexisting autoimmune disorder (found in 3.15% of Graves' disease and 4.24% of Hashimoto's thyroiditis cases).
Relative risks of almost all other autoimmune diseases in Graves' disease or Hashimoto's thyroiditis were significantly increased (>10 for pernicious anemia, systemic lupus erythematosus, Addison's disease, celiac disease, and vitiligo)."
I have hypothyroidism.
Last year my B12 was in the 300's and the neurologist said from their standpoint they want it around 800.
We began the shots for about two months and now I just take B12 sublingually - sometimes I forget about taking it but I was recently tested and am at 742.
I can't comment whether there is a link between hypo and a deficiency....just thought I would chime in my experience.
You have quite a few threads going and I haven't been able to keep up with them all.
What was the range for your vitamin B12? Yes, less than 200 is low (according to most ranges) and you probably should start out on shots to get your levels higher......
The connection between low B12 and hypo/Hashi is that most B12 deficiency is Pernicious Anemia which is autoimmune; Hashimoto's is also autoimmune...... once you have one autoimmune disease, the chances of having another are greater.
Please post whatever thyroid levels you have, and per gimel, include reference ranges, which vary lab to lab.
Hypo patients are frequently too low in the ranges for Vitamin B12, D, and ferritin, each of which is very important. Most people do better with B12 in the upper part of its range, Vitamin D around the middle of its range, and ferritin around 80 for women. Have you been tested for Vitamin D and ferritin? Also, I recall previous posts from you. Have you been tested for Free T3 and Free T4? Please post here any additional results, along with their ranges.