Postpartum Thyroiditis in Women With Euthyroid and Hypothyroid Hashimoto's Thyroiditis Antedating Pregnancy.

Context: 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 levothyroxine (LT4).

Objective: To evaluate the risk of PPT in women with HT antedating pregnancy.

Design/setting: Retrospective chart review of pregnant women with HT antedating pregnancy seen in a university hospital (2008-2017), who were followed from preconception up to 1 year after delivery.

Patients: 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.

Outcome Measures: PPT occurrence and associated clinical characteristics/risk factors.

Results: 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%; odds ratio [OR] 9.49, 95% confidence interval [CI] 4.62-19.49). Most of the women experiencing PPT in both groups were euthyroid at the time of first-trimester evaluation (39/47 eu-HT [83%] and 16/18 hypo-HT [88.9%]). Multivariate regression analysis showed eu-HT group and first-trimester euthyroidism to be positively associated with PPT occurrence (ORs 10.71 and 3.89, respectively).

Conclusion: PPT may occur in hypo-HT women on LT4 therapy, although significantly less frequently than in eu-HT women. The 4-fold higher risk of PPT in HT women maintaining euthyroidism at first -trimester of gestation suggests that the risk of PPT could be related to the amount of unaffected thyroid tissue.