A 28-year-old female had a diffuse pigmentation since her childhood. At the first visit to our clinic, she had developed hyperpigmentation and velvety thickening of the entire skin. We diagnosed the condition as generalized acanthosis nigricans from clinical symptom and histological examinations. Moreover, a laboratory evaluation, which showed positive titers of antithyroglobulin and antithyroid peroxidase antibodies, confirmed Hashimoto thyroiditis. Despite an extensive examination, no evidence of an underlying neoplastic disease was found. Although acanthosis nigricans can commonly occur on frictional areas, she had the generalized form, which is a very rare condition called generalized acanthosis nigricans. We think this case is a benign type, because the condition of generalized acanthosis nigricans had been present since childhood. However, it is difficult to exclude the possibility of a syndromic type, since it is possible that her Hashimoto thyroiditis occurred in childhood. Generalized acanthosis nigricans is extremely rare, and it is difficult to select the proper treatment. Etretinate is one of the possible treatments for generalized acanthosis nigricans.
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