Thyroid structure, function and iodine status by pregnancy trimester in Harbin, China

Thyroid disease in pregnancy can have devastating effect on the fetus. In Harbin, China, there is insufficient knowledge about the incidence of and contributing factors to thyroid disease in pregnancy. This study investigates whether urine iodine concentration (UIC), as a proxy for iodine intake, affects the thyroid structure and/or function during each trimester.

Data of 24000 pre-pregnant women were collected from January 2017 to August 2019. Serum thyroid hormone levels were measured, and thyroid ultrasonography was performed. If thyroid ultrasonography and thyroid function findings were normal before pregnancy and were abnormal after pregnancy, the current gestational age was recorded and the UIC was measured. Finally, a total of 500 participants were included in the study.

There were significant differences in the incidence of abnormal thyroid structure and function between trimester groups (p<0.05). Thyroid nodular lesions were the most common abnormal ultrasound finding, and positive thyroid peroxide antibodies (TPOAb) were the most common abnormal thyroid function test results. There were significant differences in the median UIC between trimester groups (p<0.001); the median UIC decreased with increasing gestational age. The incidence of abnormal thyroid structure or function was not significantly different across UIC groups (p>0.05).

In women from Harbin, thyroid structural or functional abnormalities commonly occur in the second and third trimesters of pregnancy. Thyroid nodular lesions and positive TPOAb are the commonest thyroid abnormalities. The median UIC significantly declines with increasing gestational age. Pregnant women in Harbin have iodine-deficient states.