Postpartum thyroiditis (PPT) is defined as the occurrence of de novo autoimmune thyroid disease accompanied by thyroid dysfunction in the first year postpartum. However, hormonal changes resembling the typical pattern of PPT have been reported to occur even in women with pregestational Hashimoto's thyroiditis (HT) on levo-thyroxine.To evaluate the risk of PPT in women with HT antedating pregnancy.Retrospective chart review of pregnant women with HT antedating pregnancy seen in a University Hospital (2008-2017), who were followed from preconception up to one year after delivery.167 women preconceptionally diagnosed with HT, and classified as hypothyroid-HT (Hypo-HT, n=98) or euthyroid-HT (Eu-HT, n=69), according to their thyroid status at the time of diagnosis.PPT occurrence and associated clinical characteristics/risk factors.PPT occurred in 65/167 women, with a rate statistically greater in the Eu-HT than in the Hypo-HT group (68.1% vs 18.4%; OR 9.49, 95%-CI 4.62-19.49). Most of the women experiencing PPT in both groups were euthyroid at the time of 1st -trimester evaluation (39/47 Eu-HT [83%] and 16/18 Hypo-HT [88.9%]). Multivariate regression analysis showed Eu-HT group and 1st -trimester euthyroidism to be positively associated with PPT occurrence (ORs 10.71 and 3.89, respectively).PPT may occur in hypothyroid HT women on LT4 therapy, although significantly less frequently than in euthyroid HT women. The four-fold higher risk of PPT in HT women maintaining euthyroidism at 1st -trimester of gestation suggests that the risk of PPT could be related to the amount of unaffected thyroid tissue.