Can we still consider thyroid hyperfunction a protective condition for the onset of thyroid cancer?

Background: Thyroid cancer is the ninth most commonly diagnosed cancer in the world, and the most common endocrine carcinoma. It was originally believed to be a rare event in patients with thyroid hyperfunction and it was reported that hyperthyroidism had a protective role against thyroid neoplasms. However, in recent years, several studies have hypothesized that differentiated thyroid carcinomas and hyperthyroidism may coexist. Our study aims therefore to evaluate the incidence of differentiated thyroid carcinomas on definitive histological examination, in patients undergoing total thyroidectomy or hemithyroidectomy with coexisting hyperfunctioning thyroid disease, to understand whether hyperthyroidism can be considered a protective condition against the onset of thyroid neoplasms. Methods: The study involved 1,449 patients underwent to thyroid surgery from 2010 to 2018 at the General Surgery Unit, Department of Surgery, University Hospital of Parma, Parma, Italy, presenting thyroid cancer at postoperative histological exam. Patients were divided in two groups based on the presence (Group A) or absence (Group B) of hyperfunction. All data were collected in a dedicated database and include demographic data, such as age and sex, preoperative cytology, date and type of surgery, postoperative diagnosis, characteristics of aggressiveness of the neoplasm and postoperative complications. For data analysis, a P value of less than 0.05 was considered statistically significant. Results: The incidence of thyroid carcinomas was lower in patients suffering from hyperfunction compared to the incidence found in non-hyperthyroid patients, both in preoperative cytological examination and in postoperative diagnosis through histological examination. Furthermore, the tumors that have developed in patients with hyperfunction had a comparable degree of aggression and invasiveness in the two groups studied. However, we have found an equal incidence of microcarcinomas and occult carcinomas on postoperative histological examination. Postoperative complications in patients with cancer were similar, regardless of the presence or absence of hyperfunctioning thyroid disease. Conclusions: Our study confirms that hyperthyroidism is a protective condition against thyroid carcinoma, but the finding of an equivalent incidence of occult carcinomas in the two groups stresses the need to perform a cytological examination in case of a nodular pathology in a hyperthyroid patient before performing a treatment.