Benign thyroid disease and dietary factors in thyroid cancer: a case-control study in Kuwait.

We conducted a population-based study of 313 case-control pairs in Kuwait to examine the aetiology of thyroid cancer, the second most common neoplasm among women in this and several other countries in the Gulf region. Among the demographic variables, individuals with 12+ years of education had a significantly reduced risk of thyroid cancer (OR=0.6; 95% CI: 0.3-0.9). The average age at diagnosis (+/-s.d.) of thyroid cancer was 34.7+/-11 years in women and 39+/-13.4 years in men. History of thyroid nodule was reported only by cases (n=34; 10.9%; lower 95% CI: 12.0); and goitre by 21 cases and four controls (OR=5.3; 95% CI: 1.8-15.3). There was no significant increase in risk with history of hypothyroidism (OR=1.8) or hyperthyroidism (OR=1.7). For any benign thyroid disease, the OR was 6.4 (95% CI: 3.4-12.0); and the population attributable risk was about 26% (95% CI: 21.1-30.9). Stepwise regression analysis showed that high consumption of processed fish products (OR=2.2; 95% CI: 1.6-3.0) fresh fish (OR=0.5; 95% CI: 0.4-0.7) and chicken (OR=1.7; 95% CI: 1.2-2.3) were independently associated with thyroid cancer with significant dose-response relationships. Among the thyroid cancer patients who reported high consumption of fish products, a large majority also reported high consumption of fresh fish (98%) and shellfish (68%). No clear association emerged with consumption of cruciferous vegetables. These data support the hypothesis that hyperplastic thyroid disease is strongly related to thyroid cancer; and that habitual high consumption of various seafoods may be relevant to the aetiology of thyroid cancer. The association with chicken consumption requires further study.