Relationship between parity and subclinical hypothyroidism in a large cohort of pregnant women without thyroid diseases

Context: Increasing parity is considered a reason for screening for thyroid dysfunction during pregnancy. Nevertheless, evidence on the relationship between parity and subclinical hypothyroidism (SCH) during pregnancy is controversial and based on studies performed outside of pregnancy. Objective: To evaluate the association between parity and SCH in the first trimester of pregnancy. Design: Cross-sectional cohort study. Setting: Prenatal care outpatient services of IRCCS Policlinico San Matteo, Pavia, Italy. Patients: Pregnant women at the first antenatal visit without known thyroid disorders, personal history of autoimmune diseases, or family history of thyroid autoimmunity. Main Outcome Measures: The rates of SCH (defined as a first-trimester TSH ≥ 4 mU/L) before and after stratification for thyroid peroxidase antibody status (TPOAb) Results: The study group included 2408 women. The rates of SCH significantly increased with parity (P < .001) for women with 2 prior pregnancies (12/166; 7.2%) or ≥3 prior pregnancies (9/60; 15.0%) when compared with nulliparous women (51/1372; 3.7%). The rates of SCH significantly increased according to parity in TPOAb-negative women (P < .001) but not in TPOAb-positive women (P = .400). Among TPOAb-negative women, the probability of having SCH increased significantly in women who had 2 prior pregnancies (odds ratio [OR], 2.112; confidence interval [CI] 1.029-4.335, P = .042) and in women with ≥3 prior pregnancies (OR, 3.476; CI, 1.224-9.873; P = .019) when compared with nulliparous women, independently of age, body mass index, previous adverse pregnancy events and smoking. Conclusion: Higher parity is associated with a higher prevalence of SCH during pregnancy, especially in TPOAb-negative women.