Incidence of postpartum thyroid dysfunction in patients with type I diabetes mellitus.

OBJECTIVE: To assess the incidence of postpartum thyroid dysfunction in women with type I diabetes. DESIGN: Cohort study of consecutive patients with clinical and biochemical assessments of thyroid dysfunction and glycemic control done during the first postpartum week and 3 and 6 months after delivery. SETTING: University medical center providing obstetric care for more than 90% of the pregnant women in the region with type I diabetes. PATIENTS: Fifty-one patients with type I diabetes who resided in the Hamilton, Ontario, catchment area and who delivered babies between May 1989 and June 1991, were not taking thyroid medication or thyroactive or immunosuppressive medications. Forty patients completed follow-up. MAIN OUTCOME MEASURES: Postpartum thyroid dysfunction was defined on the basis of thyroid function test results. RESULTS: Postpartum thyroid dysfunction occurred in 10 of 40 patients (25%; 95% CI, 12.7% to 41.2%); postpartum thyroiditis developed in 9 patients and postpartum Graves disease developed in 1 patient during the first 6 months after delivery. The incidence could have varied between 19.6% and 41.2% , depending on whether none or all of the 11 nonparticipating eligible patients developed thyroid dysfunction. Glycemic control was not affected by thyroid dysfunction. CONCLUSIONS: Women with type I diabetes are at a high risk for symptomatic postpartum thyroid dysfunction and therefore may benefit from routine thyroid function screening at postpartum visits.