Many people with autoimmune thyroiditis also have antibodies against the parietal cells that make up part of their stomachs.
The effects of having these antibodies can include a reduction in the ability to absorb vitamin B12 from food. (The parietal cells produce intrinsic factor which is crucial to normal absorption of B12. When attacked by antibodies they cannot produce enough.) This is why it might be necessary to supplement with B12 even if you are on a good diet which ordinarily would provide plenty. Typically oral supplementation with 1000 mcg methylcobalamin is used - with such a large dose you absorb it even without intrinsic factor.
Testing for B12 is desirable (and fairly readily available) but, as so often, the reference range is often argued over.
Human bodies can store enough B12 to last for several years - so you could have an acceptable B12 level when first tested and only slowly worsen. So don't rely on "I was tested for that at the beginning".
The abstract below suggests that at referral around 30% of patients had these antibodies.
Prevalence of Parietal Cell Antibodies in a Large Cohort of Patients with Autoimmune Thyroiditis.
Checchi S, Montanaro A, Ciuoli C, Brusco L, Pasqui L, Fioravanti C, Sestini F, Pacini F.
Section of Endocrinology and Metabolism, Department of Internal Medicine, Endocrinology and Metabolism and Biochemistry, University of Siena , Siena, Italy .
Background: Autoimmune thyroiditis (AIT) may be associated with other organ-specific autoimmune disorders, including autoimmune gastritis, but the prevalence of this association is not entirely quantified. The aim of this study was to investigate the prevalence of parietal cell antibodies (PCA) in a large cohort of consecutive patients with AIT. Methods: We retrospectively studied 2016 consecutive women and 258 men with AIT seen at our referral center in the period from 2004 to 2008. All patients were screened for the presence of PCA in the serum.
Results: The prevalence of serum PCA in female patients was 29.7% and progressively increased from 13% in the first-second decade of life to peak at 42% in the ninth decade. During follow up, 21.1% of the PCA-positive patients converted to PCA-negative status. Mean (±standard deviation) basal PCA levels in this group were significantly lower (32?±?28?U/mL) compared with those remaining PCA positive (129?±?200?U/mL). A similar prevalence (29.8%) with a similar age-dependency was found in male patients.
Conclusions: In conclusion, our study demonstrates a high, age-dependent prevalence of PCA in an unselected large population of patients with AIT.