Subclinical hypothyroidism, autoimmunity, and late obstetric outcomes: a population-based cohort study

The 2026 American Thyroid Association guidelines no longer use thyroid-antibody status to guide levothyroxine treatment in subclinical hypothyroidism (SCH), yet the relevance of SCH and thyroid autoimmunity to obstetric outcomes beyond early pregnancy has rarely been examined at population scale. We evaluated the association between maternal thyroid function, thyroid autoimmunity, and a composite of late obstetric complications in a population-based cohort. We conducted a retrospective, population-based cohort study of 23,499 singleton pregnancies among Clalit Health Services members in Israel (2013–2022) with at least one gestational thyroid-stimulating hormone (TSH) measurement and documented thyroid-autoantibody testing; pregnancies with persistently suppressed TSH (maximum <0.4 mIU/L) were excluded. Pregnancies were classified by gestational TSH (euthyroid, maximum 0.4–4 mIU/L; SCH, mean ≥4 and maximum <10 mIU/L) and antibody status, yielding four groups, and assessed for a prespecified composite of 14 obstetric complications. Inverse-probability weighting (IPW) balanced baseline covariates, and rate ratios (RRs; ratio of mean complication counts) were estimated using IPW-weighted quasi-Poisson regression; “any complication” was analyzed as a secondary binary outcome. After weighting, neither SCH nor thyroid autoimmunity was associated with an increased rate of composite complications. Antibody-positive euthyroid women had a modestly lower composite rate than antibody-negative euthyroid women (RR 0.92; 95% CI 0.88-0.96), as did SCH antibody-positive women versus euthyroid antibody-negative women (RR 0.93; 95% CI 0.87-0.98), although the latter attenuated after additional adjustment for parity and obstetric history. Within SCH, antibody status was not associated with the composite (RR 0.99; 95% CI 0.88-1.11), a finding stable after adjustment for recorded levothyroxine treatment, and among antibody-negative women SCH was not associated with an increased composite rate (RR 0.93; 95% CI 0.87-0.99). Healthcare-utilization patterns were consistent with, but did not establish, a surveillance effect. In this clinically detected and managed cohort, we did not observe an increased burden of late composite obstetric complications associated with SCH or thyroid autoimmunity; the modestly lower rates observed in some comparisons may reflect surveillance, survivor effects or residual confounding. These findings are consistent with current TSH-guided, guideline-based management rather than antibody-driven treatment decisions.