![]() ![]() In particular, the risk of atrial fibrillation (AF) ranged from a hazard ratio (HR) of 1.68 (95% confidence interval, 1.16-2.43) ( 8) to a relative risk (RR) of 3.1 (95% CI 1.7-5.5) ( 9), depending on the studied population as detailed in section Association Between Thyroid Function and Atrial Fibrillation. In addition to increased risk of coronary heart disease in subclinical hypothyroidism ( 7) and subclinical hyperthyroidism ( 8), there is an increased risk for arrhythmia in SCTD due to the effects of circulating thyroid hormones on cardiac function. Most patients with SCTD are asymptomatic or have non-specific symptoms (e.g., fatigue, weight loss/gain, heat/cold intolerance, poor concentration reported as ‘brain fog’). In subclinical hypothyroidism, bradycardia and diastolic hypertension may be found, whereas subclinical hyperthyroidism may be associated with tachycardia and dyspnea on exertion. Subclinical hypothyroidism is diagnosed by elevated serum levels of thyroid-stimulating hormone (TSH) with free thyroxine (T4) levels within the reference range, and subclinical hyperthyroidism is diagnosed by low TSH in conjunction with free T4 and triiodothyronine (T3) levels within reference ranges ( 4– 6). SCTD comprises both subclinical hypothyroidism and subclinical hyperthyroidism, which are diagnosed using serum blood tests. The prevalence of asymptomatic patients with SCTD is estimated to be 10-15% depending on the cohort ( 2, 3), while symptoms were more often reported by patients with hypothyroidism than those in euthyroidism although no specific symptom was sensitive enough for diagnosis ( 2). Subclinical thyroid dysfunction (SCTD) is a common condition in the general population, especially in older individuals and women, with a prevalence of 10-15% in some cohort studies ( 1– 3). In this review, we explore the interplay between thyroid hormones and atrial fibrillation, management controversies in subclinical hyperthyroidism, and potential strategies to improve the management of atrial fibrillation in patients with subclinical hyperthyroidism. Wearable devices that measure the heart electrical activity continuously may be a novel strategy to detect atrial fibrillation in patients at risk. ![]() Guidelines also recommend screening for AF in patients with known hyperthyroidism. All patients with overt hyperthyroidism should be treated, and treatment of subclinical hyperthyroidism should be considered in patients older than 65 years with TSH < 0.4 mlU/L, or in younger patients with TSH < 0.1 mlU/L. Guidelines recommend the measurement of TSH in the evaluation of new-onset atrial fibrillation. ![]() Mechanistic data from animal and human physiology studies as well as observational data in humans support an association of subclinical hyperthyroidism with atrial fibrillation. Atrial fibrillation is the most commonly diagnosed cardiac arrhythmia and has been associated with an increased risk of mortality, heart failure, stroke, and depression. Subclinical hypothyroidism is diagnosed by elevated serum levels of thyroid-stimulating hormone (TSH) with thyroxine levels within the reference range, and subclinical hyperthyroidism is diagnosed by low TSH in conjunction with thyroxine and triiodothyronine levels within reference ranges. Subclinical thyroid disorders have a high prevalence among older individuals and women.
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