Thyroid function monitoring during pregnancy in euthyroid women with thyroid autoimmunity.

Background: Thyroid autoimmunity (TAI) may be present in 1-17% of pregnant women. Monitoring of thyroid function in euthyroid pregnant women positive for anti-thyroperoxidase antibodies (TPOAb+) is recommended.

Objective: To determine the prevalence and possible clinical and biological risk factors of biochemical progression (rise in serum thyroid-stimulating hormone (TSH) > 2.5 mU/L) at second blood sampling during pregnancy, in euthyroid women (TSH ≤ 2.5 mU/L) according to their TPOAb status.

Methods: This study included demographic and biological data from two previously published cohorts (n = 274 women from August 1996 to May 1997 Copenhagen cohort, and n = 66 women from January 2013 to December 2014 Brussels cohort) having at least two measurements of TSH and free thyroxine (FT4) and at least one of TPOAb during spontaneously achieved singleton pregnancies.

Results: The majority of women studied did not show biochemical progression. Only 4.2% progressed, significantly more frequently among TPOAb+ women, as compared to TPOAb- group (9.4 vs 2.7%, P = 0.015). No rise in serum TSH > 4 mU/L at 2nd sampling was observed. Higher baseline TSH levels were associated with biochemical progression in both TPOAb+ (P = 0.05) and TPOAb- women (P < 0.001), whereas maternal age, BMI, multiparity, smoking, FT4, and TPOAb concentrations were not significantly different between women with and without progression.

Conclusions: Only a minority of euthyroid women with thyroid autoimmunity presented biochemical progression and none with a TSH > 4 mU/L. Larger studies are needed to better target the subset of women who would benefit most from repeated thyroid function monitoring during pregnancy.