Hypothyroidism incidence in and around pregnancy: A Danish nationwide study

OBJECTIVE: Immunological changes in and after a pregnancy may influence the onset of autoimmune diseases. An increased incidence of hyperthyroidism has been observed both in early pregnancy and postpartum, but it remains to be seen if the incidence of hypothyroidism varies in a similar way.

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

METHOD: All women who gave birth to a singleton live-born child in Denmark from 1999-2008 (n=403,958) were identified and data on hospital diagnosis of hypothyroidism and redeemed prescriptions of thyroid hormone were extracted. The overall incidence rate (IR) of hypothyroidism, 1997-2010, and the IR in 3-month intervals before, during and after the woman's first pregnancy in the study period were calculated and compared with the IR of hyperthyroidism.

RESULTS: Altogether 5,220 women were identified with onset of hypothyroidism from 1997-2010 (overall IR 92.3/100,000/year) and 1,572 women developed hypothyroidism in the period from 2 years before to 2 years after birth of the first child birth in the study period. The incidence of hypothyroidism decreased during the pregnancy (incidence rate ratio (IRR) versus overall IR in the rest of the study period: first trimester: 0.89 (95% CI 0.66-1.19)), second trimester: 0.71 (0.52-0.97)), third trimester: 0.29 (0.19-0.45)), and increased after birth with the highest level 4-6 months postpartum (IRR 3.62 (2.85-4.60)).

CONCLUSION: These are the first population-based data on the incidence of hypothyroidism in and around pregnancy. The incidence declined during pregnancy followed by a sharp increase postpartum. Notably, hypothyroidism as opposed to hyperthyroidism showed no early pregnancy peak.