Abstract Gigantomastia is a rare entity characterized by diffuse and excessive breast enlargement. It mainly occurs during puberty and pregnancy as a consequence of hormonal fluctuations. We report an unusual case of gigantomastia in a 29‐year‐old woman with a history of personal and familiar autoimmune phenomena. She had autoimmune thyroiditis and several positive autoantibodies, and developed 3 crises of the disease, 1 related to pregnancy (possibly hormone‐mediated), and 2 unrelated to pregnancy in which an autoimmune role is raised based in clinical, histological and laboratory findings. Immunological aspects that may be involved in this presentation of the disease are discussed.