Among pregnant women at low risk for thyroid dysfunction, the existence of subclinical hypothyroidism, thyroid autoimmunity or both prior to 8 weeks’ gestation increases the likelihood of miscarriage, according to recent findings.

In the prospective cohort study, researchers evaluated 3,315 women seen at the gynecology/obstetrics clinics of 13 hospitals and six prenatal clinics in three Chinese cities. The women were enrolled in the study between January and September of 2012 at the clinics, all of which were located in areas of China deemed iodine sufficient. Women identified for analysis were between 4 and 8 weeks’ gestation, and had lived in the area for more than 5 years.

Study participants completed questionnaires pertaining to demographic and obstetric history (maternal age, gestational age, previous pregnancies/births, and previous miscarriage). The questionnaires also elicited information about education level, income, smoking status, alcohol consumption, family history of thyroid disorders, history of type 1 diabetes, autoimmune diseases and treatments involving head or neck irradiation.

At baseline, all women were imaged by ultrasound to confirm ongoing pregnancy. The study’s primary outcome was miscarriage, which was characterized as sudden pregnancy loss before 20 weeks’ gestation. Participants underwent monthly thyroid function tests and ultrasound imaging at the clinics. The thyroid tests measured thyroid stimulating hormone, free thyroxine, thyroid peroxidase antibody, thyroglobulin antibody and urinary iodine.

On the basis of the thyroid test results, women were classified into four groups: euthyroidism (ET), isolated subclinical hypothyroidism (SCH), isolated thyroid autoimmunity (TAI) and subclinical hypothyroidism with TAI (SCH and TAI). Patients in the SCH cohort were further divided into two subgroups SCH1 and SCH2, based on TSH levels; as a result, the SCH and TAI group was also further stratified into two groups (SCH+TAI1 and SCH+TAI2).

The researchers found that compared with ET women (2.2%), those with SCH had a significantly higher risk of miscarriage (7.1%; adjusted OR=3.40; 95% CI: 1.62-7.15). Additionally, women with TAI alone were also at significantly increased risk of miscarriage (5.7%) vs. ET patients (2.2%; adjusted OR=2.71; 95% CI, 1.43-5.12), as were women in the TAI+SCH1 groups (10%; adjusted OR=4.96; 95% CI: 2.76-8.90) and the TAI+SCH2 groups (15.2%; adjusted OR=95% CI, 3.76-24.28).

Among 110 women who miscarried, the gestational ages were lower among those with subclinical thyroid disorders (11.3 weeks) vs. ET women (9.33 weeks; P =.024). The subgroup analysis also determined that higher TSH levels were associated with lower gestational age at miscarriage, in women with or without TAI.

The researchers said these findings may support the need to monitor thyroid function in pregnant women.

“To some extent, these finding support universal screening of women of reproductive age for thyroid function and thyroid autoimmunity early during pregnancy or during the preconception period,” the researchers wrote.