Context: Knowledge is scarce concerning the significance of thyroid dysfunction/antibodies during pregnancy in regard to pregnancy complications/later maternal morbidity. Objective: The aim of this study was to evaluate the association between maternal thyroid dysfunction/antibodies during pregnancy and pregnancy complications or later maternal hypertension, diabetes, and thyroid disease. Design and Setting: We studied a prospective population-based cohort, Northern Finland Birth Cohort 1986 (NFBC 1986), with follow-up of 20 yr. Medication and hospital discharge records were used to assess maternal morbidity to hypertension, diabetes, and thyroid diseases. Participants: The study consisted of mothers of NFBC 1986 with early pregnancy serum samples for thyroid function and antibody analyses (n = 5805). Mothers were grouped and compared according to these test results. Main Outcome Measures: We focused on preeclampsia and gestational diabetes during index pregnancy, later maternal hypertension, diabetes, and thyroid disease morbidity and total mortality. Results: Thyroid dysfunction and antibodies were not associated with pregnancy complications. Overt hypothyroidism was associated with subsequent maternal thyroid disease hazard ratio (HR) (95% confidence interval), 17.7 (7.8–40.6) and diabetes 6.0 (2.2–16.4). Subclinical hypothyroidism 3.3 (1.6–6.9), TPO-Ab-positivity 4.2 (2.3–7.4), and TG-Ab-positivity 3.3 (1.9–6.0) were also associated with later thyroid disease. No association was found between thyroid dysfunction/antibodies and hypertension or overall mortality. Conclusions: Thyroid dysfunction and antibodies during pregnancy seem to predict later thyroid disease. Overt hypothyroidism poses risk of diabetes. Thyroid dysfunction and antibodies during pregnancy seem to predict later maternal morbidity to thyroid diseases.