Reproductive factors and risk of thyroid cancer. A prospective study of 63,090 women from Norway

Abstract

Summary This prospective study of 63,090 Norwegian women with 124 cases of thyroid cancer diagnosed during 1961 -1989 (McTiernan et al., 1984;Ron et al., 1987;Preston-Martin et al., 1987;Franceschi et al., 1990a). In autopsy studies, no significant sex and age contrasts in the frequency of occult microcarcinomas seem to be present (Fukunaga & Yatani, 1975) On the basis of the total number of cases included in any particular analysis, the expected number was found for each level of the study variable, assuming no association with cancer (Thomas & Gart, 1983 Times of death and emigration were taken into account in the calculation of expected numbers (Tarone, 1975). The analyses were adjusted for age at the start of follow-up (with 5-year age groups), county and in special cases other demographic and reproductive variables. The adjustments were made by forming a stratum for each combination of covariables. The analyses also produced two-tailed P-values for linear trend. Stratified logistic regression analyses were carried out according to the procedure described by Thomas and Gart (1983). In the estimation procedure, a correction for death and emigration was introduced by decreasing the initial number at risk by half the number of such events occurring among those who did not develop thyroid cancer. Due to missing values for certain reproductive variables, the number of cases varied somewhat between analyses. The majority of the women aged 49 years or less at the start of the follow-up were premenopausal at the time of interview. Consequently, the information on number of births will be incomplete for some participants in this age category. Therefore, in addition to the analyses that included all age groups, separate analyses were performed based on the participants aged 49 years or less and those aged 50 or more at the start of follow-up. Separate analyses were also carried out in subgroups according to age at diagnosis, < 54, 55-69, >70 years. Table I shows the distribution of cases according to demographic variables. The risk of thyroid cancer was highest in the counties of Aust-Agder and Tr0ndelag, whereas no urban/rural gradient was observed. Increased risk was observed in the occupational category fishing, ships officers, crew (odds ratio estimate, based on stratified logistic regression: 2.14, 95% CI: 1.30-3.54 for the total series). Results Demographic factors

Cohort study of thyroid cancer in a San Francisco Bay area population

“…In agreement with our results, a study among 63,090 Norwegian women revealed no strong associations with reproductive factors. 38 The lack of association with reproductive factors is also consistent with a report among Los Angeles County women 39 and with pooled analyses of case-control studies of thyroid cancer. 40,41 However, some modest associations have also been reported, including bilateral oophorectomy, 39 early first childbirth (before age 20 or Ͻ5 years after menarche), artificial menopause 42 and miscarriage as outcome of first pregnancy.…”

Epidemiology of reproductive and hormonal factors in thyroid cancer: Evidence from a case‐control study in the Middle East

“…29,38,39,41 Moreover, a decreased risk was found in a study from Italy for women who had ever experienced an abortion (RR ϭ 0.6; 95% CI: 0.4 -1.0). 25 Our data showed an inverse relationship between miscarriage and risk of thyroid cancer, particularly for women who had a miscarriage as outcome of first pregnancy and for those who have had Ն3 miscarriages; there was also a significant trend in decreasing risk with increasing number of miscarriages.…”

“…28 A positive association, of similar magnitude, with late last birth (Ն35 years) was also demonstrated in a prospective cohort study of 63,090 women in Norway. 29 Among the various reproductive factors, parity has received most attention. Most case-control studies have demonstrated that women with thyroid cancer have had relatively more pregnancies or live births than controls.…”

“…Another cohort study of 63,090 Norwegian women with 124 cases of thyroid cancer, however, did not find any increase in risk with parity. 29 It is plausible that pregnancy at young age induces some protective modification in risk and this threshold is overwhelmed by prolonged and repeated hormonal (TSH, hCG and sex hormones) stimulation due to large number of pregnancies and subsequent metabolic stress and hypertrophy of the thyroid gland. It is also possible that thyroid gland of older women, who have also borne a relatively large number of children, may become more susceptible to some of the carcinogenic stimuli occurring during pregnancy.…”

Reproductive Risk Factors for Thyroid Cancer: A Prospective Cohort Study in Khon Kaen, Thailand

“…Thus, it is known that exposure to ionizing radiation, especially in childhood, increases the risk of developing thyroid cancer (Silverman, 1984;IARC, 2001;Zielinski et al, 2009;Furukawa et al, 2013) However, it is still not clear why thyroid cancer is markedly predominant in females. Previous observational studies on the reproductive system have produced conflicting results (Akslen et al, 1992;Pham et al, 2009;Horn-Ross et al, 2011;Kabat et al, 2012). We, therefore, investigated potential risk factors for thyroid cancer using data from the Khon Kaen Cohort Study, with the main objective of determining the effect of reproductive factors on the occurence of thyroid cancer.…”