Parity is associated with increased thyroid volume solely among smokers in an area with moderate to mild iodine deficiency

Objective: Pregnancy has been suggested as part of the explanation of the gender difference in the prevalence of goitre, but opposing results have been reported on the association between pregnancy and goitre. We investigated the association between parity and thyroid volume and a possible impact of iodine deficiency and tobacco smoking on this association. Design: A comparative, cross-sectional study of 3712 women randomly sampled from the general population in two geographical areas with moderate and mild iodine deficiency. Methods: The participants answered questionnaires with an obstetric anamnesis, and ultrasonography of the thyroid was performed. Data were analysed in linear models and logistic regression analysis to adjust for age, iodine status, use of oral contraceptives and smoking habits. Women with present or recent pregnancies were excluded from the analyses. Results: A higher thyroid volume was found among parous than among nulliparous women (P=0.007). The association between parity and thyroid volume was strongest in the youngest age groups, in the region with the most severe iodine deficiency, and among smokers. No association was found between parity and the prevalence of solitary or multiple thyroid nodules. Number of births, age at menarche or menopause, the number of fertile years, and age at first childbirth were not associated with thyroid volume. Conclusion: Pregnancy increases thyroid volume, particularly when combined with tobacco smoking and iodine deficiency. The effect is probably reversible seen over a spectrum of several years.