Hashimoto's Thyroiditis Effects on Papillary Thyroid Carcinoma Outcomes: A Systematic Review

Papillary thyroid carcinoma (PTC) and Hashimoto's thyroiditis (HT), also known as chronic lymphocytic thyroiditis, are both common thyroid diseases that are increasing in prevalence. PTC is well-differentiated cancer that generally has an excellent prognosis. HT is an autoimmune disease that often leads to hypothyroidism. A significant proportion of PTC patients also have HT. This systematic review will analyze the effect of HT on the characteristics and outcomes of PTC. Several databases were systematically searched using relevant medical subject headings (MeSH) keywords and phrases examining the connection between PTC and HT and the effect of their coexistence. Inclusion and exclusion criteria were applied, followed by quality appraisal. After that filtration process, 23 articles were selected with a total of 41,646 patients. Out of 22 studies commenting on tumor size, 12 studies demonstrated it to be smaller in HT patients, while 10 studies observed no effect. Eleven studies examined PTC angioinvasion, most of which found no difference in HT and non-HT patients. However, two studies found angioinvasion to be reduced in PTC patients. As for capsular infiltration, out of the five studies commenting on it, two found decreased occurrence, one found increased occurrence, and two had no difference. Extrathyroidal extension was found to be reduced in seven studies out of the 14 that examined it. Six other studies saw no effect. One study found increased extrathyroidal extension incidence overall, and another found it to be the case in patients younger than 45 years of age. Lymph node metastases were found to be reduced in several studies, while others found no difference. One study found increased central lymph node metastases in HT patients. As for prognoses, most studies found positive aspects. One study found an increased recurrence rate in HT patients, however, it did not have a relationship with deaths. In conclusion, when managing HT or HT and PTC patients, HT patients should be monitored closely for suspicious nodules due to their frequent co-occurrence. Although the effect of HT on PTC has been shown to be mostly protective, multifocality is more common in those patients and, therefore, a total thyroidectomy should be favored. The high false positive rates of lymph node metastases in diagnostic methods should be kept in mind when considering lymph node dissection. Additional diagnostic procedures such as frozen section histology should be considered for verification.