Effect of Levothyroxine Treatment on Incident Dementia in Adults with Atrial Fibrillation and Hypothyroidism

BACKGROUND AND OBJECTIVE: The possible adverse or positive effects of levothyroxine treatment among patients with atrial fibrillation have been debated. Atrial fibrillation, levothyroxine treatment, and dementia are common among older adults, yet little is known about the effects of levothyroxine on the development of dementia. METHODS: The study population included all adults (n = 12,057) aged ≥ 45 years with a diagnosis of atrial fibrillation at 75 primary care centers in Sweden during 2001–2007 without an earlier recorded dementia diagnosis. Patients with a diagnosis of hypothyroidism and levothyroxine prescription (n = 756; 180 men and 576 women), or with a prescription without this diagnosis (n = 415, 100 men and 315 women), were compared to patients without a diagnosis and prescription (n = 10,886). Cox regression was performed, with hazard ratios and 95% confidence intervals, with outcome defined as dementia of all causes until 31 December, 2010. RESULTS: During a mean 5.7 years (standard deviation 2.4 years) of follow-up, a total of 749 patients with atrial fibrillation (6.2%) were diagnosed with dementia. Women with hypothyroidism and a levothyroxine prescription showed lower hazard ratios (0.61; 0.41–0.90) in adjusted Cox regression models (age, socioeconomic factors, co-morbidity, and warfarin treatment). Levothyroxine prescription without hypothyroidism was non-significant. CONCLUSIONS: We found that women with atrial fibrillation with levothyroxine treatment and hypothyroidism have a lower risk of incident dementia than women with atrial fibrillation without levothyroxine treatment and no hypothyroidism. The results should be confirmed in larger studies. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s40261-018-0740-3) contains supplementary material, which is available to authorized users.