Objectives: Thyroid cancer accounts for 2% of all cancers in industrialized countries but is the most common endocrine cancer. There is a marked worldwide geographic variation in thyroid cancer incidence, the highest incidence rates being observed in New Caledonia, and a constant increase of thyroid cancer incidence has been observed during the past several decades. Although the evolution in medical practice probably accounts for some of the increase, other factors such as environmental and/or lifestyle factors may also play a role. However, there are few established risk factors of thyroid cancer apart from ionizing radiation exposure in childhood. We aimed to investigate the etiologic role of a family history of thyroid cancer, of hormonal and reproductive factors, anthropometric factors and environmental and occupational exposures in thyroid cancer. Methods: Analyses mainly rely on a population-based case-control study conducted in metropolitan France (“CATHY study”). Data of a case-control study conducted in New-Caledonia were also used to investigate the role of a family history of thyroid cancer as a risk factor for thyroid cancer in this area where thyroid cancer incidence is very high. The CATHY study is based on 621 cases diagnosed from 2002 to 2007 in three administrative areas: the Calvados, the Marne and the Ardennes, and 705 controls matched to cases by age, sex and residence area. The New Caledonian case-control study is based on 332 cases diagnosed form 1985 to 1999 and 412 controls matched to cases by age and sex. Results: A family history of thyroid cancer or multinodular goiter in first degree relatives was associated with an increased risk of thyroid cancer in France as well as in New Caledonia. Among hormonal and reproductive factors, a later age at menarche, a younger age at menopause and a elevated number of full-term pregnancies were associated with an increased risk of thyroid cancer. An inverse relationship was observed between the cumulative number of menstrual cycles and risk of thyroid cancer. Oral contraceptive use was associated with a lower risk of thyroid cancer, and an inverse relationship was observed between oral contraceptive use duration and risk of thyroid cancer. Height, body mass index and body surface area were positively associated with thyroid cancer risk. Analyses by occupation showed a higher risk of thyroid cancer for workers in leather industry, in plastics and rubber industry, in papermaking, in wood manufacturing, in automotive repair and in metallurgy.
