An extranodal marginal zone B-cell lymphoma arising from mucosa-associated lymphoid tissue in the thyroid gland. The vast majority of cases are associated with chronic lymphocytic thyroiditis. The disease is Monarch Disease Ontology term MONDO_0006464 (thyroid gland mucosa-associated lymphoid tissue lymphoma). Also known as: thyroid MALT lymphoma, thyroid mucosa-associated lymphoid tissue lymphoma.