Prevalence and features of abdominal ET. The specific prevalence of ET is approximately 1 per 100,000-300,000 persons[3]. Abdominal tumors arise because of various reasons, and the diagnosis of abdominal tumors is very difficult. ET is most commonly found in the neck, and abdominal ET is relatively rare. We reviewed the English literature in PubMed and summarized the prevalence and features of abdominal ET.
Adrenal gland ET
Guerra et al[4] reported that adrenal gland thyroid is very rare and is difficult to understand based on thyroid embryology. However, on the basis of the reported cases[5–14], the adrenal gland is not an uncommon site of abdominal ET. The cause of adrenal gland ET remains unknown. Although some diseases including adenocarcinoma are associated with adrenal masses with cystic lesions, dermoid cysts and cortical adenoma have been reported, ET should be considered when the adrenal gland shows both normal hormonal data and a cystic lesion.
Gallbladder ET
ET is occasionally found in the gallbladder. Campora et al[15] reported that only 3 cases of gallbladder ET have been described. However, to the best of our knowledge, at least 6 case reports on this disease existed in the literature[16–21]. Gallbladder ET generally has no specialized symptoms but is occasionally associated with recurrent right abdominal pain. ET could be observed in the wall of the gallbladder.
Pancreatic ET
Although ET could be found along the descending glands, ET in the pancreas is rare. To the best of our knowledge, 3 studies have been reported on pancreatic ET[22–24]. All the patients were middle-aged women, and all lesions had sizes of approximately 70*30 mm. One of the three patients had no symptoms with a diagnosis of pancreatic cancer[22], one had recurrent pain in the right upper quadrant with a diagnosis of neuroendocrine neoplasm[24], and one had a complaint about dyspeptic symptoms with a diagnosis of a duodenal ulcer[23].
Liver ET
ET in the liver rarely occurs. Only three cases of ET in the liver[25, 26] and porta hepatis[3] have been reported. Additionally, a German report was also found in the references when we conducted out literature search[27]. All patients were women. Since the liver is an organ easily metastasized by cancers, ruling out thyroid cancer metastasis before the diagnosis of ET is important. Moreover, whether ET is combined in the liver metastases is key. Kondo et al[25] reported a mimicking ET in a 48-year-old woman with follicular carcinoma of the thyroid. However, the author also discussed that the liver tumor may not be an ET but rather an incidentally detected liver metastasis.
Gastric ET
ET is considered as normal thyroid tissue in the gastric mucosa and along the gastrointestinal tract[4]. However, distinguishing between metastatic thyroid cancer and normal thyroid tissue inside the stomach is important. ET was discovered in the stomach of a 35-year-old man without a thyroid tumor who suffered from gastric pain for six months. Biopsies were taken, and the microscopy sections showed thyroid tissue composed of colloid material and follicular cells[28].
Other sites
The other sites of abdominal ET included the duodenum[29], mesentery[30], appendix[31], gynecological organs (ovary, uterus and fallopian tube)[32–34] and others. Although few cases of other sites of abdominal ET have been reported, struma ovarii is a noteworthy disease because this condition may be easily misdiagnosed, and determining benign disease from malignant disease is difficult. Struma ovarii is diagnosed on the basis of thyroid tissue in the ovarian structures. Most patients with struma ovarii are asymptomatic, and the condition is incidentally found on ultrasonography or CT. The pathological diagnosis of our patient also suggested that struma ovarii should be excluded, based on further examinations and the combination of those results with clinical symptoms; the patient was ultimately diagnosed with ET.
Diagnosis. The mechanism of how ET migrates into the abdomen is not fully understood. Cassol et al[19] reported that ET in the gastrointestinal tract, liver and pancreas could be explained as a heteroplastic or metaplastic phenomenon because these locations and the thyroid share a common embryologic origin from the foregut endoderm. The thyroid gland tissue is composed of two cell types, the C cells and the thyroid follicular cells[28]. Romero-Rojas et al[2] showed that the lack of C cells in histology and immunohistochemical profiles is one of the important diagnostic criteria.
Iodine-131 or technetium-99m pertechnetate have been employed to discover ET and is based on the typical characteristics of thyroid tissues uptaking radioisotopes. Most patients were admitted to the hospital after the abdominal mass was accidentally found. Nonetheless, radioactive examinations are rarely performed for patients without symptoms. CT scans, B-mode ultrasonography and magnetic resonance imaging (MRI) are the main imaging tools for abdominal ET. Fine needle aspiration cytology (FNAC) provides correct diagnoses at a rate higher than 95% and is considered the most accurate diagnostic method. FNAC is a very useful diagnostic tool when ET is not identified, especially before a surgery. Occasionally, intraoperative frozen pathology is also an effective method for some suitable patients.