Hypothyroidism and pregnancy loss: comparison with hyperthyroidism and diabetes in a Danish population-based study.

Background: Hypothyroidism is a common endocrine disease. The frequency of pregnancy loss in women with known hypothyroidism as opposed to women with a later diagnosis of hypothyroidism has not been evaluated and compared with other common endocrine diseases.

Design: Population-based cohort study using Danish nationwide registers.

Participants: All pregnancies in Denmark, 1997-2008, resulting in live birth (n = 732 533), spontaneous abortion (n = 112 487) or stillbirth (n = 2937) were identified together with information on maternal hypothyroidism, hyperthyroidism and diabetes.

Methods: Cox model was used to estimate adjusted hazard ratio (aHR) with 95% confidence interval (95%CI) for spontaneous abortion and stillbirth, reference: no hypo- or hyperthyroidism or diabetes (n = 824 310).

Results: We identified 4951 pregnancies where maternal hypothyroidism was diagnosed before the pregnancy (group 1) and 2464 pregnancies where maternal hypothyroidism was diagnosed in the 2-year period after the pregnancy (group 2). In group 1, 825 pregnancies (16·7%) resulted in spontaneous abortion which was more frequent than in nonexposed (13·2%), (aHR 1·19 (95%CI 1·12-1·27)), and of the same magnitude as in hyperthyroidism (17·2%, P = 0·5) and diabetes (17·5%, P = 0·2) diagnosed before the pregnancy. In group 2, the frequency was 12·2% (aHR 0·92 (0·84-1·02)). In group 2, 16 pregnancies (0·65%) resulted in stillbirth which was more frequent than in nonexposed (0·36%), (aHR 1·81 (1·11-2·97)), of the same magnitude as in hyperthyroidism (0·82%, P = 0·5) and less frequent than in diabetes (2·9%, P < 0·001) diagnosed after the pregnancy. In group 1, the frequency was 0·40% (aHR 1·11 (0·68-1·82)).

Conclusions: Hypothyroidism increased the risk of both early and late pregnancy loss as did hyperthyroidism and in particular diabetes. We hypothesize that undetected or insufficiently treated maternal disease in the pregnancy may be of causal importance.