Parity as a thyroid size-determining factor in areas with moderate iodine deficiency

Among the factors that may influence thyroid size, pregnancy and its goitrogenic effect have been widely investigated, but thyroid volume and pregnancy have never been compared retrospectively, and there are no data on the possible relationship between thyroid size and parity. The purpose of this work was to evaluate the effects of pregnancy on thyroid volume in a moderate iodine deficiency area, to assess the possibility of a relationship between thyroid size and parity status in healthy females. A group of 208 nongoitrous healthy women underwent thyroid volume estimation by ultrasound examination. All subjects were euthyroid and negative for thyroid autoantibodies. They were assigned to different groups, according to the number of completed pregnancies. Five groups were formed (0, 1, 2, 3, 4 or more term pregnancies). Mean thyroid volume increased progressively among the groups: group 0 (14.8 ± 0.7 mL); group I (16.0 ± 0.9 mL); group II (17.1 ± 0.6 mL); group III (18.2 ± 0.6 mL); group IV (20.3 ± 0.9 mL). The increment in thyroid volume was statistically significant between group 0 and groups III (P <0.01) and IV (P <0.001), and also between group I and group IV (P <0.05). No independent effect of body weight and age-on thyroid volume was seen. Our results indicate that, in an area with moderate iodine deficiency, the goitrogenic effect of pregnancy is not fully reversible. Moreover, the statistically significant increase in thyroid volume, observed in relation to parity, is the first clinical demonstration of a cumulative goitrogenic effect of successive pregnancies, providing a strong argument to increase the iodine supply during pregnancy, even in conditions with moderate iodine deficiency.