Iodine deficiency and thyroid cancer trends in three regions of Thailand, 1990–2009

• Pooled papillary cancer incidence significantly increased between 1990 and 2009.
• Pooled follicular cancer incidence significantly decreased between 1990 and 2009.
• The largest changes occurred in Thailand’s historically iodine-deficient regions.
• Iodine exposure and supplementation may affect late-stage thyroid carcinogenesis.

BackgroundIodine deficiency may play a role in thyroid cancer carcinogenesis. Because Thailand has region-specific historical iodine deficiency, it is ideal to evaluate the potential impact of recent national iodine supplementation policies on thyroid cancer incidence trends.MethodsWe examined thyroid cancer trends in Thailand from 1990 to 2009 in three geographically separated populations (Songkhla Province [south], Chiang Mai Province [north], and Khon Kaen Province [northeast]), each with a different historical prevalence of iodine deficiency. We used Joinpoint analysis and age-period-cohort (APC) models to investigate trends in thyroid cancer incidence.ResultsPooled incidence of papillary cancers significantly increased (Males APC: 2.0, p < 0.05; Females APC: 7.3 [1990–2001, p < 0.05], −2.1 [2001–2009]) and incidence of follicular cancers significantly decreased (Males APC: −5.2, p < 0.05; Females APC: −4.3 [1990–1998, p < 0.05], 12.3 [1998–2001], −17.0 [2001–2005, p < 0.05], 8.2 [2005–2009]) in both males and females between 1990 and 2009. The largest increases in papillary cancer incidence, and the largest decreases in follicular cancer incidence, occurred in historically iodine-deficient regions. Interestingly, the significant histological changes coincided with Thailand’s most recent national iodination policy. The thyroid cancer trends in females were better explained by period effects than cohort effects.ConclusionsThis study adds to the research indicating that papillary carcinoma incidence increases, and follicular carcinoma incidence decreases, as population-level iodine deficiency declines, and suggests that iodine exposure may affect late stages of thyroid carcinogenesis. However, our findings are limited by the ecological study design and lack of data prior to iodine supplementation.