Background: Iodine 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.
Methods: We 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.
Results: Pooled 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.
Conclusions: This 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.