A pooled analysis of case-control studies of thyroid cancer. IV. Benign thyroid diseases.

Objective: To obtain more precise estimates of the association between thyroid cancer and benign thyroid diseases and to elucidate the role of potential confounders or effect modifiers.
Methods: The original data from 12 case-control studies from the United States, Asia, and Europe were pooled. Based on 2,094 women and 425 men with cancer of the thyroid and, respectively, 3,248 and 928 control subjects, odds ratios (ORs) and the corresponding 95% confidence intervals (CIs) were obtained by conditional regression models, conditioning on study and age at diagnosis, and adjusting for age and radiotherapy.
Results: A history of hypothyroidism was not associated with cancer risk (pooled ORs = 0.9, 95% confidence interval, CI: 0.7-1.3 in women and 1.7, 95% CI: 0.3-11.7 in men). ORs for hyperthyroidism were 1.4 (95% CI: 1.0-2.1) in women and 3.1 (95% CI: 1.0-9.8) in men. In women, however, risk was lower in the absence of or after allowance for history of goiter. Pooled ORs for a history of goiter were 5.9 (95% CI: 4.2-8.1) in women and 38.3 (95% CI: 5.0-291.2) in men. Risk for a history of benign nodules/adenomas was especially high (OR = 29.9, 95% CI: 14.5-62.0, in women; 18 cases versus 0 controls in men). The excess risk for goiter and benign nodules/adenomas was greatest within 2-4 years prior to thyroid cancer diagnosis, but an elevated OR was present 10 years or more before cancer.
Conclusions: Goiter and benign nodules/adenomas are the strongest risk factors for thyroid cancer, apart from radiation in childhood.