Table 1.

Summary of epidemiologic evidence on risk factors for thyroid cancer

	Association direction	Evidence of consistency across studies and dose-response	Bias potential (epidemiologic studies)
Ionizing radiation	↑ Childhood exposure only	Consistent findings; evidence of dose-response	Some; exposure, if known, may be associated with thyroid screening
Benign thyroid conditions	↑ Thyroid nodules ↑ Goiter ↑ Hyperthyroidism − Hypothyroidism	Consistent findings; some evidence of dose-response for thyroid function parameters	Substantial; conditions likely associated with thyroid screening
Dietary factors	↑ Iodine (papillary thyroid cancer) ↓ Iodine (follicular thyroid cancer) − Vegetables or fruits − Meat − Coffee or tea	Inconsistent findings; no clear evidence of dose-response (except iodine and follicular thyroid cancer)	Moderate (iodine); confounding likely in ecological studies
Metabolic factors	↑ Obesity/excess adiposity − Metabolic syndrome − Diabetes Type II − Hypertension − Dyslipidemia	Consistent findings; evidence of dose-response (obesity/excess adiposity)	Some; conditions may be associated with thyroid screening
Reproductive factors/sex steroid hormones	− Parity, age at menarche, age at menopause, use of exogenous hormones ↑ Hysterectomy ↑ Infertility	Inconsistent findings; no clear evidence of dose-response with hormonal exposures	Moderate; reproductive events or hormone use may be associated with thyroid screening
Perinatal and in utero exposures	↑ Birth weight − Maternal or paternal age − Pregnancy characteristics and complications − Maternal smoking ↑ Maternal thyroid conditions ↑ Congenital hypothyroidism	Consistent findings and evidence of dose-response (birth weight); evidence on maternal and congenital thyroid conditions based on only one study	Moderate; maternal/paternal characteristics and medical conditions may be associated with thyroid screening
Cigarette smoking and alcohol consumption	↓	Consistent findings; evidence of dose-response	Some; behavior may be associated with thyroid screening
Environmental contaminants	− Flame retardants − Pesticides − Volcanic environment/heavy metals	Inconsistent findings; no clear evidence of dose-response	Some; exposure, if known, may be associated with thyroid screening

Association with thyroid cancer risk, based on overall evidence: ↑ = positive; ↓ = inverse; − = null