Here on PAS and over on Thyroid UK (and, I assume, in most related groups) we see a considerable number of people who actually have, or suspect they have, aspects both of thyroid and of B12 issues.
On my home turf of Thyroid UK, I often recommend people hop over here and read or discuss their possible B12 issues. From time to time, I also mention Thyroid UK to posters here. This has made me realise how rarely the medical establishment ever links the two. Yet this is not because there is no evidence, nor that the link is dismissed, but that the link does not reach those who need to know in their professional roles.
Complete achlorhydria occurs in more than half of myxedematous patients. As many as 25 percent of patients with myxedema, like those with Hashimoto’s thyroiditis, have circulating antibodies directed against the gastric parietal cells. This finding explains, at least in part, the frequency of achlorhydria and impaired absorption of vitamin B12. It is reported that up to 14 percent of patients with idiopathic myxedema have coincident pernicious anemia.
In case anyone who has Graves is hoping to escape:
It is possible that thyrotoxicosis may increase the need for vitamin B12, as shown experimentally, and perhaps for folic acid. Also, there is an increased incidence of antigastric antibodies and mild pernicious anemia in patients with Graves’ disease.
These are not quoted from a renegade, "alternative" source - but a very conventional site which has been written by some of the more respected thyroid experts.