Iodine supplementation in Sweden and regional trends in thyroid cancer incidence by histopathologic type

Abstract

We studied regional patterns of thyroid cancer incidence in relation to iodine intake and iodinization in Sweden using 5,838 incident cases diagnosed in the period 1958-1981. Region was defined either by iodine status, urban-rural status or healthcare region (internal controls). Age, period and cohort models were fitted to evaluate trends and regional variation in incidence by histopathologic type. In iodine-deficient areas, the relative risk (RR) of developing thyroid cancer was 0.92 for all histologic types combined, 0.80 for papillary cancer and 0.87 for anaplastic carcinoma. Residence in iodine-deficient regions was associated with a 2-fold increased risk of follicular cancer in men (RR 1.98) and a 17% increase in risk in women (RR 1.17).Regional differences in iodine intake fell after iodinization of the food supply, which was started in 1936 and enhanced in 1966.Nevertheless, incidence of both papillary and follicular carcinoma increased during the study period, with largely similar trends in iodine-deficient and iodine-sufficient areas. Overall, residence in urban or rural areas was not an important determinant of incidence, though trends in the incidence of papillary, follicular and anaplastic cancer did vary between urban and rural areas. The occurrence of thyroid cancer differed only marginally between the 6 health-care regions in Sweden, suggesting that the observations in regions defined by iodine intake were unlikely to be artifactual. Our data suggest that iodinization of the food supply is not associated with adverse trends in the occurrence of thyroid cancer.

Urinary iodine is increased in papillary thyroid carcinoma but is not altered by regional population iodine intake status: a meta-analysis and implications

“…Other epidemiological studies reported that increased levels of iodine intake caused the increased TC incidence and the proportion of PTC after universal salt iodization (USI). Improved TC diagnosis may be one of the causes of the increased TC incidence [10], and the decreased FTC and UTC incidence may increase significantly the proportion of PTC [4,5,11]. Therefore, we support that high iodine intake is not a risk factor of PTC either with a large single dose iodine administration or long-term high iodine intake.…”

“…It has been confirmed that iodine excess-induced thyroid dysfunction may lead to benign thyroid disease, such as autoimmune thyroiditis, benign thyroid nodules (BTN), hypothyroidism and hyperthyroidism [3]. But, it remains considerable controversy about the relationship between high iodine intake and thyroid cancers (TC) risk [4][5][6][7][8][9][10][11][12][13].…”

“…Over the past few decades, TC incidence has rapidly increased, PTC predominated in the histological types of TC after universal salt iodization (USI) [4][5][6]14]. It is considered that increasing iodine intake has contributed to the increase in PTC incidence, whereas it is debatable [4][5][6][7][8][9][10][11][12][13].…”

Thyroid cancer incidence by histological type and related variants in a mildly iodine‐deficient area of Northern Italy, 1998 to 2009

“…However, recent studies have shown that the serum levels of TSH are in fact not increased or even lower in persons with mild‐to‐moderate iodine deficiency 31. Conversely, PTC is more prevalent in iodine‐sufficient areas32 and its frequency increases after iodine prophylaxis,33 a phenomenon also accompanied by a decrease in PTC size and an attenuation of malignant phenotype 34‐36. Iodine deficiency‐associated thyroid cancer has been associated with a specific molecular basis; RAS mutation in particular was found to be more frequently present in FTC from iodine‐deficient areas than that diagnosed in regions with sufficient iodine intake 37.…”

Thyroid Cancer, Thyroiditis and Dietary Iodine: A Review Based on the Salta, Argentina Model

“…As a matter of clarification for cases with medullary carcinoma whose mean age was similar to those with papillary cancer (Table 3), thyroiditis occurred in three (23.07%) of 13 patients aged 43, 47, and 75 from the latest post-prophylaxis period, respectively, while none occurred in the three patients studied from the previous periods. Previous observations showed a wide variation as to the frequency of Hashimoto's thyroiditis in glands with thyroid cancer (e.g., <1-23%), used different criteria to define the thyroiditis as either focal or Hashimoto's, and also included papillary microcarcinomas which were probably incidental a Two primary thyroid lymphomas b Four primary thyroid lymphomas Table 9 Papillary:follicular carcinoma ratio in endemic goiter areas before and/or after iodine prophylaxis [14,[35][36][37][38][39][40][41][42][43] Before [30]. The latter contention was suggested to explain the high prevalence of RET mutation in Hashimoto's thyroiditis [101], molecular results that have been interpreted differently or even disputed by others [102,103].…”