Incidence of Subclinical Hypothyroidism and Hypothyroidism in Early Pregnancy

Untreated and subclinical hypothyroidism (SCH) has been associated with adverse pregnancy complications such as increased risk of miscarriage, hypertension, preeclampsia, and preterm delivery. However, in Sweden, screening for thyroid dysfunction during pregnancy is only recommended for women with a high risk of thyroid disease. Therefore, the aim of this study was to determine the incidence of clinical and SCH in women in the first trimester of pregnancy.

In this prospective study, 1298 pregnant women were divided into three groups: one unselected general screening group (n = 611), one low-risk group comprising women without risk factors for thyroid disorder (n = 511), and one high-risk group comprising women with an inheritance or suspicion of thyroid disease or undergoing treatment for thyroid disease (n = 88). Serum was obtained up to gestational week 13, and thyrotropin (TSH) was analyzed.

The incidences of thyroid dysfunction in the three screening groups were 9.8% in the general screening group, 9.6% in the low-risk group, and 10.2%, p = 0.948, in the high-risk group. In the women with known hypothyroidism on levothyroxine treatment, 50.6% had serum TSH levels above 2.0 mIU/L.

High-risk screening is not useful in predicting which women are at risk of thyroid disease in early pregnancy since ∼10% of women with SCH or hypothyroidism could not be diagnosed in this way.