Thyroid volume changes during pregnancy and after delivery in an iodine-sufficient Republic of Slovenia

Abstract 

Objective

Literature data concerning thyroid enlargement during pregnancy are not conclusive. Our aim was to systematically follow the thyroid volume changes during pregnancy and after delivery in an iodine-sufficient area.

Study design

Prospective study of healthy pregnant women living in an iodine-sufficient area. We followed 118 pregnant women with the mean age 30.9±4.1 years in the first trimester (mean 11.2±2.5 weeks of pregnancy), in the third trimester (mean 31.6±1.7 weeks of pregnancy), and 4 months after delivery (mean 15.9±3.9 weeks). Additionally, 71 women were also evaluated 14 months after delivery (mean 13.3±1.1 months). All women were negative for thyroid autoantibodies. We measured urinary iodine concentration (UIC), thyroid volume, serum TSH, and body mass index (BMI). After delivery, in a subgroup of women we also estimated the colour flow Doppler sonography (CFDS) patterns 0, I, II and III, where thyroid vascularity increased from pattern 0 to III, and the peak systolic velocity (PSV) using a 7.5mHz linear transducer.

Results

Median UIC in the third trimester (176μg/g creatinine) was significantly higher than 4 and 14 months after delivery (P=0.030, P<0.001, respectively). Thyroid volume in the third trimester (11.3±3.1mL) was significantly greater (P<0.001) than in the first trimester (8.7±2.5mL), 4 months after delivery (8.6±2.5) and 14 months after delivery (7.8±2.4mL). TSH concentration was significantly higher in the third trimester than in the first trimester and 4 months after delivery (P=0.007, P=0.006, respectively). As expected, BMI was the highest in the third trimester. CFDS pattern I was more frequent 4 months after delivery than 14 months after delivery (P<0.001). Similarly, PSV was significantly higher 4 months after delivery than 14 months after delivery (P<0.001). Linear regression analysis revealed TSH and BMI as significant independent predictors for thyroid volume.

Conclusion

In an iodine-sufficient area, thyroid volume increases during pregnancy and decreases after delivery, and the changes in volume are associated with changes in TSH and BMI. They may be viewed as indicators for metabolic and haemodynamic changes during pregnancy.

Keywords: Pregnancy, Thyroid volume, Thyrotropin, Body mass index, Intrathyroidal blood flow