Letter to the Editor Strong Correlation of Abnormal Serum and Urinary Iodine Levels with Papillary Thyroid Cancer: A Case-control Study*

Thyroid cancer is the most common malignancy of the endocrine system, and its incidence is increasing worldwide. In China, its incidence has increased from 1.78/10 in 1988 to 10.58/10 in 2013. Thyroid cancers are mainly classified into papillary thyroid cancer (PTC), follicular thyroid cancer (FTC), medullary thyroid cancer (MTC), and undifferentiated or anaplastic thyroid cancer (ATC) according to the histological subtypes. Among them, PTC is the most common subtype, comprising about 80%–90% of all thyroid cancers. Iodine is indispensable to humans because it is essential for thyroid hormone synthesis. There is a U-shaped relationship between iodine intake and thyroid disease, either a deficiency or excess of iodine can lead to thyroid disease. However, the role of iodine intake in thyroid cancer remains uncertain. Liu et al. have found that high iodine consumption may decrease the risk of thyroid cancer in iodine-deficient areas. Kim et al. investigated 215 PTC patients and found that relatively low iodine intake and excess iodine intake are risk factors for the development of PTC in an iodine-replete area. Zhao et al. have found that urine iodine concentration less than 100 μg/L group was inversely associated with multifocality, while excess iodine intake was independently associated with an increased risk of larger tumor size in female PTC patients. However, most of these studies used only urinary or serum iodine levels, and their results were diverse. The clinical significance of serum and urinary iodine levels and the tumorigenesis and clinicopathological features of PTC, such as whether it is associated with multifocality and lymph node metastasis, remain controversial. In the current study, a total of 845 patients with PTC, 310 patients with benign thyroid nodules, and 374 control group were recruited. We aimed to investigate the correlation of serum and urinary iodine levels with the risk of developing PTC, and the clinicopathological characteristics of patients with PTC as per the iodine status were also compared. Blood and urine samples of 845 PTC patients and 310 benign thyroid nodule patients were obtained from the Harbin Medical University Cancer Hospital before any medical treatment such as cytoreductive surgery, radiotherapy and chemotherapy. According to the appropriate protocols, patients with an initial diagnosis of thyroid nodule were enrolled from November 2015 to March 2018. The PTC patients and benign thyroid nodule patients were enrolled based on post-operative pathology and excluded them with abnormal liver or kidney functions. Blood and urine samples of control group were obtained from the Second Affiliated Hospital of Harbin Medical University. The control group were recruited from the health check up clinic and comprised 374 ageand sex-matched participants without thyroid diseases or evidence of other diseases, such as other cancers or other liver or kidney diseases. Data collection from the human subjects and study protocols were approved by the Institutional Ethics Committees of the Harbin Medical University, and signed, informed consent was obtained from all