Does relatively low iodine intake contribute to thyroid cancer? An ecological comparison of epidemiology

Environmental factors related to the origin and evolution of differentiated thyroid cancer: a narrative review

INTRODUCTION

The global incidence of thyroid cancer (TC) has increased in the last decades. While improvements in diagnosis may contribute, overdiagnosis is also a possibility. This review focuses on the epidemiology, risk factors, and immune microenvironment associated with differentiated TC (DTC).

AREAS COVERED

A search was conducted in Scielo, Scopus, and EMBASE databases, involving 72 articles. TC is the most common endocrine neoplasm, with DTC form being predominant. Its incidence has globally risen, particularly among women aged over 45. Endogenous risk factors for DTC include genetic disorders, race, age, female gender, obesity, and type 2 diabetes mellitus. Environmental risks involve ionizing radiation, whether through therapeutic treatment or environmental contamination from nuclear accidents, iodine deficiency, endocrine disruptors, residence in volcanic areas, environmental pollution, and stress. The use of anti-obesity medications remains controversial. The tumor's immune microenvironment is the histological space where tumor cells interact with host cells, crucial for understanding aggressiveness. Immunotherapy emerges as a promising intervention.

EXPERT OPINION

Recent advances in DTC management offer transformative potential, requiring collaborative efforts for implementation. Emerging areas like precision medicine, molecular profiling, and immunotherapy present exciting prospects for future exploration, shaping the next era of diagnostic and therapeutic strategies in thyroid cancer research.

Iodine nutrition and papillary thyroid cancer

Thyroid cancer (TC) is the most frequent endocrine malignancy. The incidence of TC, especially papillary thyroid carcinoma (PTC), has continued to rise all over the world during the past few years, for reasons that are not entirely clear. Though the phenomenon of overdiagnosis is occurring, it is not the sole driver of the substantial increase in incidence. Lifestyle, environmental factors, or complications are considered to be potential risk factors. Among these factors, iodine is a micronutrient that is vital to thyroid function. The effect of iodine intake on PTC has been controversial for many years and the epidemiological or experimental studies provided diametrically opposite conclusions. Combining all these studies, we found that iodine nutrition may affect the overall prevalence, distribution of the histological types, and clinicopathological aggressiveness of TC, especially PTC. However, the available evidence is poor due to the impact of various internal and external related factors. Therefore, this article sums up available results from both epidemiological and experimental studies, future studies are also warranted to expound on the relationship between overall PTC prevalence and iodine intake.

Time for Re-Evaluating the Human Carcinogenicity of Ethylenedithiocarbamate Fungicides? A Systematic Review

Background. In January 2021, the European Union ended the license of Mancozeb, the bestselling ethylenedithiocarbamate (EBDC) fungicide, because of some properties typical of human carcinogens. This decision contrasts the IARC classification of EBDC fungicides (Group 3, not classifiable as to human carcinogenicity). A systematic review of the scientific literature was conducted to explore the current evidence. Methods. Human and experimental studies of cancer and exposure to EBDC fungicides (Mancozeb, Maneb, Zineb, and others) and ethylene thiourea (ETU), their major metabolite, published in English as of December 2021, were retrieved using PubMed, the list of references of the relevant reports, and grey literature. Results. The epidemiological evidence of EBDC carcinogenicity is inadequate, with two studies each suggesting an association with melanoma and brain cancer and inconsistent findings for thyroid cancer. Experimental animal studies point at thyroid cancer in rats and liver cancer in mice, while multiple organs were affected following the long-term oral administration of Mancozeb. The mechanism of thyroid carcinogenesis in rats has also been shown to occur in humans. Genotoxic effects have been reported. Conclusions. The results of this systematic review suggest inadequate evidence for the carcinogenicity of EBDC fungicides from human studies and sufficient evidence from animal studies, with positive results on three out of ten key characteristics of carcinogens applying to humans as well. An IARC re-evaluation of the human carcinogenicity of EBDC fungicides is warranted.

Insufficient iodine nutrition may affect the thyroid cancer incidence in China

Epidemiological studies have focused on the effects of iodine intake on the risk of thyroid cancer. However, their relationship is still obscure. The objective of the present study was to examine the association in the Chinese population. A new ecological study which combined the Data of Annual Report of Cancer, the Survey of Iodine Deficiency Disorders (IDD) surveillance and the Water Iodine Survey was conducted to analyse the relationship between iodine intake and the thyroid cancer incidence in China. In total, 281 counties were included. Thyroid cancer incidence was negatively correlated with the consumption rate of qualified iodised salt (CRQIS) and positively correlated with goiter prevalence (GP) of children aged 8-10 years, residents' annual income and coastal status. Areas with a low CRQIS and areas with a high GP had a relatively high incidence of thyroid cancer. Regression models showed that a low CRQIS and a high GP in children aged 8-10 years (both reflecting iodine deficiency status) play a substantial role in thyroid cancer incidence in both males and females. Additionally, living in coastal areas and having a high annual income may also increase the risk of thyroid cancer. These findings suggest that mild iodine deficiency may contribute to the exceptionally high incidence of thyroid cancer in some areas in China. Maintaining appropriate iodine nutrition not only helps to eliminate IDD but also may help to reduce the occurrence of thyroid cancer.

Analysis of the correlation between high iodized salt intake and the risk of thyroid nodules: a large retrospective study

BACKGROUND

Currently, whether daily excess iodized salt intake increases the risk of thyroid nodules and even thyroid cancer remains controversial. Our research group aimed to provide a theoretical basis for the clinical guidance of daily iodized salt intake and the prevention of thyroid nodules through a retrospective analysis of the correlation between daily iodized salt intake and the risk of thyroid nodules and thyroid cancer in Hunan, China.

METHODS

This study retrospectively analyzed the data of subjects who underwent a physical examination at the Health Management Center, Third Xiangya Hospital of Central South University, between January 1, 2017, and December 31, 2019. Subjects enrolled in this study underwent thyroid ultrasonography and tests to urine routines and liver and kidney function, and all subjects completed a questionnaire survey. The daily iodized salt intake of the study subjects was estimated based on spot urine methods (Tanaka). A multivariate logistic regression model was used to analyze the relationship between daily iodized salt intake and thyroid nodules and thyroid cancer.

RESULTS

Among the 51,637 subjects included in this study, the prevalence of thyroid nodules was 40.25%, and the prevalence of thyroid cancer was 0.76%; among all enrolled subjects, only 3.59% had a daily iodized salt intake less than 5 g. In addition, we found that a daily intake of more than 5 g of iodized salt was not only an independent risk factor for the occurrence of thyroid nodules (odds ratio (OR): 2.08, 95% confidence interval (CI): 1.86-2.31, p < 0.001) but also an independent risk factor for the occurrence of thyroid cancer (OR: 5.81, 95% CI: 1.44-23.42, p = 0.012). A pooled analysis showed a significantly higher risk of thyroid nodules in subjects aged > 60 years with a daily iodized salt intake of more than 5 g compared to subjects aged < 60 years with a daily iodized salt intake of no more than 5 g (OR: 4.88, 95% CI: 4.29-5.54, p < 0.001); the risk of thyroid cancer was not significantly different between subjects aged > 60 years with a daily iodized salt intake of more than 5 g and those aged < 60 years with a daily iodized salt intake of no more than 5 g (OR: 2.15, 95% CI: 0.52-8.95, p = 0.281). The risk of thyroid nodules was not increased in physically active subjects with a daily iodized salt intake of more than 5 g compared to physically inactive subjects with a daily iodized salt intake of no more than 5 g (OR: 1.12, 95% CI: 0.97-1.28, p = 0.111). The same protective effect of physical activity was observed for thyroid cancer in subjects whose daily iodized salt intake exceeded 5 g. The risk of thyroid nodules was reduced for subjects with an education level of postgraduate and above, even when the daily iodized salt intake exceeded 5 g, compared to those with high school education and below and a daily iodized salt intake of no more than 5 g (OR: 0.79, 95% CI: 0.66-0.93, p = 0.005); however, a protective effect of education level on the occurrence of thyroid cancer was not observed. Independent risk factors affecting daily iodized salt intake greater than 5 g included age, triglycerides, family history of tumors, physical activity, and marital status.

CONCLUSIONS

Daily intake of more than 5 g of iodized salt increased the risk of thyroid nodules and thyroid cancer, while increased physical activity and education level reduced the risk of thyroid nodules and thyroid cancer caused by iodized salt intake.

Exposure to 2.45 GHz radiofrequency modulates calcitonin-dependent activity and HSP-90 protein in parafollicular cells of rat thyroid gland

In this study we analyzed the response of parafollicular cells in rat thyroid gland after exposure to radiofrequency at 2.45 GHz using a subthermal experimental diathermy model. Forty-two Sprague Dawley rats, divided into two groups of 21 rats each, were individually exposed at 0 (control), 3 or 12 W in a Gigahertz Transverse Electro-Magnetic (GTEM) chamber for 30 min. After radiation, we used simple or fluorescence immunohistochemistry to measure calcitonin cells or cellular stress levels, indicated by the presence hyperplasia of parafollicular cells, heat shock protein (HSP) 90. Immunomarking of calcitonin-positive cells was statistically significant higher in the thyroid tissue of rats exposed to 2.45 GHz radiofrequency and cell hyperplasia appeared 90 min after radiation at the SAR levels studied. At the same time, co-localized expression of HSP-90 and calcitonin in parafollicular cells was statistically significant attenuated 90 min after radiation and remained statistically significantly low 24 h after radiation, even though parafollicular cell levels normalized. These facts indicate that subthermal radiofrequency (RF) at 2.45 GHz constitutes a negative external stress stimulus that alters the activity and homeostasis of parafollicular cells in the rat thyroid gland. However, further research is needed to determine if there is toxic action in human C cells.

Geographical influences on thyroid abnormalities in adult population from iodine-replete regions: a cross-sectional study

The studies on the increasing incidence of thyroid abnormalities are scarce. The aim of this current study was to ascertain the effects of geographical region on thyroid abnormalities under the context of universal salt iodization (USI). We randomly selected 1255 participants residing in inland and 1248 in coast, with the determination of urinary iodine concentration (UIC) and functional and morphological abnormalities of thyroid gland. The median UIC was significantly higher for the inland participants (188.5 μg/L) than the coastal participants (128.5 μg/L; p < 0.001), indicating iodine sufficiency in both populations according to the recommended assessment criteria by the World Health Organization. However, the spectrum of thyroid abnormalities varied between regions, with hypothyroidism prevalent in inland and thyroid nodules in coast. The associations between region and thyroid abnormalities via binary logistic regression models showed that the coastal participants were at a higher risk of total thyroid abnormalities than those from the inland (OR 1.216, 95% CI 1.020-1.449), after the adjustment of ten confounders (demographical characteristics, smoking status, metabolism syndrome, and hyperuricemia). These results indicated that further investigations of the adverse effects of hypothyroidism and thyroid nodules on health burden is urgently needed to sustain USI program.

Association between excessive chronic iodine exposure and the occurrence of papillary thyroid carcinoma

The aim of the present study was to elucidate the association between excessive chronic iodine exposure and the risk of developing papillary thyroid carcinoma (PTC). The demographic information and pathological characteristics of patients with thyroid nodules were retrieved from medical records at The Second Hospital of Shandong University. A fasting urine specimen was collected, and creatinine and urinary iodine concentration (UIC) were determined. The water iodine data from the domicile districts of these patients were collated from published reports. The results revealed that almost half of the patients with PTC (44.3%) also exhibited a high UIC (≥300 µg/l). Multivariate analysis revealed that the adjusted odds ratio for high UIC was 3.987 (95% CI: 1.355–11.736) and the adjusted area under the receiver operating characteristic curve was 0.776 (95% CI: 0.687–0.864), which was associated with PTC risk in patients with thyroid nodules. Integrated ecological assessment of chronic iodine exposures demonstrated that >80% (81.4%) of the patients with PTC who also exhibited a high UIC were from historically non-iodine-deficient regions, and 66.7% of patients with PTC who resided in historically iodine-excessive regions were characterized by high UICs. Importantly, a high UIC was significantly associated with capsular invasion and extrathyroid metastasis (P<0.05). Moreover, self-matching results indicated that, in patients with PTC, there were no significant differences in UIC grading between the pre- and postoperative specimens. In conclusion, excessive chronic iodine exposure is significantly associated with the risk of PTC, which contributes to increased capsular invasion and extrathyroid metastases. However, further research is required to validate these findings and to elucidate the potential molecular mechanisms involved.

Prophylactic lymph node dissection in clinically N0 differentiated thyroid carcinoma: example of personalized treatment

Considering the 'differentiated thyroid carcinoma (DTC) epidemic', the indolent nature of DTC imposes a treatment paradigm shift toward elimination of recurrence. Lymph node metastases in cervical compartments, encountered in 20-90% of DTC, are the main culprit of recurrent disease, affecting 5-30% of patients. Personalized risk-stratified cervical prophylactic lymph node dissection (PLND) at initial thyroidectomy in DTC with no clinical, sonographic or intraoperative evidence of lymph node metastases (clinically N0) has been advocated, though not unanimously. The present review dissects the controversy over PLND. Weighing the benefit yielded from PLND up against the PLND-related morbidity is so far hampered by the inconsistent profit yielded by PLND and the challenging patient selection. Advances in tailoring PLND are anticipated to empower optimal patient care.