In autoimmune gastritis (AIG) or thyrogastric syndrome (new one to me), iron deficiency may come before B12/cobalamin deficiency. Iron levels may need to be monitored, but AIG patients have difficulty with supplements and could need infusions. Iron deficiency + thyroid problem may be a signal for the gastric condition that will create pernicious anemia. Pernicious anemia is the end stage. I hope this helps with recent comments about not giving treatment until the B12 anemia shows. Below, I will try to give the web link but in case it doesn't work, also the article.
Toh, B.H. (2014) Diagnosis and classification of Autoimmune Gastritis. Autoimmune Review 13(4-5). ncbi.nlm.nih.gov/pubmed/244....
Cellini, H. et al (2017). Hashimoto's thyroiditis and autoimmune gastritis. Front. Endocrinol 8 (92). (Lausanne). ncbi.nlm.nih.gov/pmc/articl....
Goncalves et al. (2014). AIG presenting as iron deficiency in children. World Journal Gastroenterology 20(42). ncbi.nlm.nih.gov/pmc/articl...