Using data from a large health plan, we performed a cohort study of thyroid cancer among 204,964 persons (aged 10 -89 at baseline in 1964 -1973, 54% female) followed for a median of 20 years. Thyroid carcinoma is rare, representing 0.5% of cancers among U.S. men and 1.7% cancers among U.S. women, but remains the most common malignancy of the endocrine system. 1 Its incidence increased from 1987 to 1991 in the United States, 2 is greater in women than in men and peaks between the ages of 25 and 65. 3,4 The prognosis of well-differentiated, localized thyroid tumors is extremely good. 1 Persons considered to be at high risk include those with a history of radiation treatment during infancy and childhood, 5,6 those with exposure to nuclear testing 7 or to nuclear reactor accidents 8 and those with benign conditions of the thyroid gland. 9 -13 Other putative determinants include diet (notably iodine deficiency or excess), 14,15 hormonal, reproductive 16 -20 and genetic factors. 21,22 Other less well-studied potential risk factors include smoking, alcohol intake, 23 body size and weight gain. 18,24 In this report, we evaluated several potential predictors of thyroid cancer (including demographic, behavioral, medical and reproductive factors as well as occupational exposures) in a large, well-defined, multi-ethnic cohort of men and women enrolled in a health-maintenance organization (HMO) in northern California and followed for up to 33 years.
MATERIAL AND METHODS
Study design and proceduresWe used a historical cohort study design with measurement of characteristics at baseline (1964 -1973) and prospective follow-up of events from baseline through the end of 1997. The cohort was composed of 95,058 male and 112,838 female subscribers of the Kaiser Permanente Medical Care Program of Northern California, aged 10 -89, who underwent multiphasic health checkups between 1964 and 1973 in San Francisco or Oakland. Of the original cohort, 28 women and 6 men were excluded for having a prior diagnosis of thyroid cancer before the multiphasic checkup. An additional 2,395 women and 503 men were excluded for having a self-reported history of thyroid operation (though the extent of surgical removal was unknown). Thus, the final sample of persons free of thyroid cancer and with an intact thyroid gland consisted of 94,549 men and 110,415 women.Northern California Kaiser Permanente members constitute an ethnic and socio-economic mix that is generally representative of the local population, except that the extremes of wealth and poverty are under-represented. 25 If study participants attended more than 1 checkup, only the data from the first were used.The multiphasic checkup collected information on several demographic factors (age, gender, race/ethnicity and educational attainment), anthropometric measurements, health behaviors (cigarette smoking and alcohol consumption), physiological characteristics (including reproductive factors), medical conditions and occupational exposures using standardized instruments and procedures previously...
