Endocrine Abstracts

Background: Whether combination therapy of levothyroxine and liothyronine increases the risk of atrial fibrillation remains a topic of debate.

Objectives: The objective of this study was to investigate the risk of atrial fibrillation in individuals with hypothyroidism treated with a combination of levothyroxine and liothyronine.

Methods: We conducted a retrospective cohort study using individual-level linkage of Danish nationwide registries to identify women who claimed a prescription of liothyronine between 2017 and 2019. These cases were matched 1:1 with women treated with levothyroxine only, based on age-group, Charlson Comorbidity Index (CCI), educational level, and year of prescription. The primary outcome was a diagnosis of atrial fibrillation, and cases and controls were compared by use of Poisson regression models to estimate the incidence rate ratios (IRRs).

Results: This retrospective cohort study included 2,896 women who claimed a prescription of liothyronine, median age 53 years (interquartile range 45 to 60). Most women (98.9%) had no major comorbidities (CCI = 0), and 87.5% had a high school or higher education. The women were followed for a median time of 2.9 years, totaling 18,361 person-years of observation time. Six women treated with liothyronine (6.7 per 10,000 person-years) were diagnosed with atrial fibrillation, compared to four in the control group (4.2 per 10,000 person-years). However, no statistically significant increased risk of atrial fibrillation was found (Incidence rate ratio of 1.61 [95% CI 0.45 – 5.70])

Conclusions: The results of this study suggest that the risk of atrial fibrillation among women taking liothyronine is low, and the addition of liothyronine compared to levothyroxine treatment alone was not associated with an increased risk of atrial fibrillation. However, caution should be exercised in interpreting these results due to the relatively short follow-up period and the small number of events observed in this age-group of women.