ObjectiveAbout one out of two women with primary hypothyroidism has to increase the dosage of exogenous levothyroxine (l-T₄) during pregnancy. Considering the detrimental impact of IVF on thyroid function, it has been claimed but not demonstrated thatl-T₄dose adjustment may be more significant in hypothyroid women who become pregnant after IVF.DesignRetrospective cohort study.MethodsHypothyroid-treated women who achieved a live birth through IVF were reviewed. Women could be included if thyroid function was well compensated withl-T₄before the IVF cycle (i.e., serum TSH <2.5 mIU/l and serum free T₄within the normal range). Serum TSH and dose adjustment were evaluated at five time points during pregnancy. The trimester ranges for serum TSH considered as reference to adjustl-T₄therapy were 0.1–2.5 mIU/l for the first trimester, 0.2–3.0 mIU/l for the second trimester, and 0.3–3.0 mIU/l for the third trimester.ResultsThirty-eight women were selected. During the whole pregnancy 32 women (84%; 95% CI: 72–96%) required an increase in the dose ofl-T₄. In most cases (n=28), this occured within the first 5–7 weeks of gestation (74%, 95% CI: 58–85%). At 5–7 weeks of gestation, the median (interquartile range) increase ofl-T₄dose for the whole cohort was 26% (0–50%). At 30–32 weeks, it was 33% (14–68%). In order to identify predictive factors of dose adjustment, we compared women who did (n=28) and did not (n=10) adjustl-T₄dosage at 5–7 weeks' gestation. Significant differences emerged for thyroid autoimmunity prevalence and for the distribution of hypothyroidism aetiology.ConclusionsThe vast majority of hypothyroid-treated women who achieve pregnancy through IVF need an increase in thel-T₄dose during gestation. This requirement tends to occur very early during gestation.