Endocrine Abstracts

Background: Overt hypothyroidism (OH) has repeatedly been associated with a number of potential lethal conditions. However, it is still debated whether OH is linked with increased mortality. Moreover, a link between OH and mortality could be the result of common genetic and environmental factors affecting both OH and mortality. Investigating twin pairs discordant for OH can minimize this potential confounding. Our objective was to investigate, at a nationwide level, whether OH influence mortality.

Study populations: A random 5% sample of the Danish population (n=281 549) and all Danish twins (n=96 064) born between 1870 and 1990.

Methods: Unpaired and intrapair Cox regression analyses were compared. From the discordant twin pair design, early environmental and genetic confounding was controlled by design. The participants were followed until December 31, 2008.

Results: In the 5% sample of the Danish population, mortality was increased by 20% in subjects with OH (Hazard ratio, HR =1.20, 95% CI, 1.14–1.27). The impact of OH on mortality remained significant after adjusting for the degree of comorbidity (HR =1.16, 95% CI, 1.10–1.23). OH was not associated with an increased mortality in the analyses of twins discordant for OH (HR =1.03, 95% CI, 0.75–1.42). Stratifying for zygosity yield essential similar results in monozygotic (HR =1.00, 95% CI, 0.54–1.83) and dizygotic same sex pairs (HR =1.04, 95% CI, 0.71–1.52).

Conclusion: Our findings of lack of association between OH and mortality within DZ and MZ twin pairs discordant for OH, implies, that the association found in the singletons may be due to genetic and environmental confounding.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported, however funding details are unavailable.