MONDO_0006980 (struma ovarii) is an ovarian mature teratoma characterized by the presence of aberrant thyroid tissue. The aberrant thyroid tissue shows morphologic changes identical to thyroid adenoma or carcinoma. Patients may present with abdominal mass and unusual symptoms due to thyrotoxicosis, or Meigs syndrome (ascites and pleural effusion). Also known as: struma ovarii (morphologic abnormality), struma ovarii NOS (morphologic abnormality).