Study population
This study included outpatient clinic patients of the Thyroid Center at Samsung Medical Center(SMC), Seoul, South Korea from November 2011 to June 2016. Research team doctors actively recruited eligible case and control patients. The Division of Endocrinology and Metabolism and Department of Surgery were congregated together at the Thyroid Center, and the research interviewers waited on-site every day and conducted the comprehensive risk factor survey whenever patients were recruited.
The case group included patients aged 20-80, newly diagnosed with pathologically confirmed PTC, with or without follicular variant, who did not have any history of cancer. Those who were found not to have cancer according to the pathologic report after the surgery were excluded. Further excluding those who did not finish the interview (n = 17) and those who were younger than 20 years-old (n = 2), a total of 1,172 cancer patients completed the interview. Of those, we included in this study 492 cancer patients who actually consented to donate urine samples. There was no significant difference in basic characteristics between urine donors and non-donors in case group, except for the proportion of men(21.7% among donors, 27.2% among non-donors, p = 0.039) and the proportion of cases with history of benign thyroid diseases(21.2% among donors, 34.8% among non-donors, p < 0.001). The control group included patients aged 20-80 with benign thyroid diseases, such as thyroid nodules, cysts, hyperplasia, hypothyroidism, hyperparathyroidism, etc. or self-induced thyroid screening, who did not have any history of cancer. As healthy as possible patients were selected as controls and many of the controls had no plans for long term follow-up. When a control patient was diagnosed with PTC later in time during the study period, he/she was switched to the case group. After excluding those who did not finish the interview (n = 29) and those who were younger than 20 years-old (n=2), a total of 1,171 control patients completed the interview. Of those, we included in this study 595 controls who actually consented to donate urine samples. There was no significant difference in basic characteristics between urine donors and non-donors in control group, except the proportion of controls with history of benign thyroid diseases(34.3% among donors, 44.0% among non-donors, p = 0.001). (Figure 1).
Data collection
Research interviewers explained the purpose and contents of the survey to the patients, obtained written consent, and conducted interviews using a structured questionnaire on various risk factors including life-style, medical history, radiation, and diet. For the cancer case group, the interview was conducted before surgery. Weight and height were measured in shoeless state using an automatic height and weight machine. After the interview was completed, participants were led to the Diabetes Education Center to measure body composition using InBody3.0(BioSpace). Ten ml whole blood and 12 ml spot urine samples were collected in accordance with the next diagnostic test schedule in the SMC’s central lab. Fasting time before sample collection was 4 hours for afternoon appointments to 12 hours for morning appointments. Collected samples were stored at -20°C on-site and then delivered every day to the Seegene central lab, the collaborating commercial lab for our research group, and then aliquoted and stored at -70°C.
Questionnaires were doubly entered into the database by two interviewers independently, and the data was compared using a Statistical Analysis System (SAS) ver.9.4. When the program picked out inconsistencies in the data, we went back to the original questionnaire and made necessary corrections to minimize the input errors.
Body mass index (BMI) was calculated by dividing the measured weight(kg) by the square of the height(m2). Smoking status was divided into nonsmokers, past smokers, and current smokers after asking if they had smoked more than 20 packs of cigarettes. The daily intake of alcohol was calculated based on the frequency and amount of alcohol intake over 1 year and the types of alcohol consumed, such as makgeolli(Korean rice wine), wine, soju, beer, and spirits. Supplement intake combined the intake frequencies of multivitamins, vitamin C, vitamin E, vitamin D, calcium, omega 3, and red ginseng. Physical activity was classified as yes or no according to whether regular exercise enough to sweat on the body was performed. Education level was classified elementary school or less, middle school diploma, high school diploma, or college degree or higher. Family history of cancer included cancer in parents and children, and history of thyroid disease included thyroid nodules and benign tumors, hypothyroidism, hyperthyroidism, goiter, and other thyroid diseases.
UIC was measured from a spot urine sample by inductively coupled plasma-mass spectrometry (ICP-MS, Perkin Elmer, ICPMSD, Waltham, MA, USA), and creatinine level was measured by Jaffe(C702, Roche, Mannheim, Germany), and the iodine/creatinine level ratio(μg/gCr) was used as the creatinine-adjusted UIC to minimize diurnal and day to day variation 22. Still, as mentioned before, UIC had limitations in reflecting long-term usual dietary iodine intake in individual level. Therefore, rather than using absolute cut-off points, such as 100μg/L for deficiency, we used the quartiles of the creatinine-adjusted UIC in the controls (<159.3 μg/gCr, 159.3–394.3 μg/gCr, 394.3–1037.3 μg/gCr, and ≥1037.3 μg/gCr) to categorize the participants into four groups.
Statistical analyses
Categorical variables were expressed as the frequency and percentage, and the chi-square test was used to compare the characteristics of the participants between case group and control group. Continuous variables were presented as the mean and standard deviation(SD), and t-test was performed to compare the continuous variables between the two groups.
Age, sex, educational level, physical activity, supplement intake, BMI, daily alcohol intake, smoking status, family history of cancer, and history of thyroid disease were adjusted for as confounding variables. Odds ratios(ORs) and 95% confidence intervals(CIs) of the risk of thyroid cancer were calculated for participants exposed to the higher creatinine-adjusted UICs(μg/gCr) compared with those exposed to the lowest creatinine-adjusted UIC using unconditional logistic regression. To evaluate possible interaction by gender and age, stratified analyses were also performed. P for interaction was calculated by adding product interaction term to the model, and results were presented under the tables. All analyses were conducted using R, version 3.6.3, and a two-tailed p value of <0.05 was defined as significant.
This study was approved by the institutional review board of Samsung Medical Center (IRB No. 2011-11-025, 2011-11-076). Informed consent was obtained from all subjects and their legal guardians. All methods were performed in accordance with the relevant guidelines and regulations.