When Hashimoto's Thyroiditis Masks Biermer's Disease: A Revealing Case of Autoimmune Polyendocrinopathy.

Biermer's disease is an autoimmune disorder characterized by atrophic gastritis leading to vitamin B12 deficiency, often revealed late due to non-specific symptoms. It may be associated with other autoimmune pathologies, notably Hashimoto's thyroiditis, as part of a type 3b autoimmune polyendocrinopathy (AIP). We report the case of a 44-year-old female patient, followed for autoimmune hypothyroidism, in whom vitamin B12 deficiency was detected. The patient showed no suggestive signs such as glossitis, paresthesia, or other neurological abnormalities. Investigations confirmed Biermer's disease, with no neurological or digestive involvement. Treatment with oral vitamin B12 rapidly corrected the deficiency. While intramuscular vitamin B12 is commonly preferred in autoimmune-related deficiencies, oral supplementation was chosen in this case and proved effective, despite not being the standard approach. This case illustrates the value of systematic vitamin B12 screening in patients with autoimmune thyroiditis and, conversely, the need to monitor thyroid function in any patient with Biermer's disease.