Atrial fibrillation (AF) is the most common arrhythmia in clinical practice and accounts for at least a third of hospitalisations due to cardiac rhythm disturbance.¹ Paroxysmal and persistent AF affects 2.2 million Americans and six million Europeans.^1,2 Patients with AF also have increased risk of impaired cognition,³ quality of life and exercise capacity, left ventricular dysfunction, and five-fold increase risk of stroke, resulting in increased health provision cost.^4,5

Thyroid dysfunction is one of the disabling treatable risk factors for AF.^6,7 Recent data have shown that overt hyperthyroidism and subclinical hyperthyroidism, which has the prevalence of 10-25% in elderly patients,^7,8 have been associated with significant increased risk of AF.⁸ Current guidelines advise aggressive treatment for patients with subclinical hyperthyroidism,⁹ so as to avoid other complications (for example, impaired ventricular relaxation, reduced exercise tolerance and increased risk of atherosclerosis, hence infarction and cardiovascular mortality) associated with subclinical thyroid disease.^10,11

Aim

This study aimed to assess the prevalence of thyroid disease in patients presenting with AF in our area. It also aimed to assess the management of their abnormal thyroid status.

Methods 

Conservative presentations of patients to hospital with known or new paroxysmal, persistent and permanent AF over a period of 12 months were recorded. Initial data was retrieved from the Hospital Inpatient Enquiry (HIPE) system. Further data was collected from patients’ clinical notes, the laboratory database and from general practitioners. Data collected included age, sex, thyroid function status and co-morbidities.

Thyroid status was assessed by the level of thyroid stimulating hormone (TSH). Euthyroidism was defined by thyrotropin levels from 0.27 to 4.2mIU/L and subclinical hyperthyroidism when thyrotropin was less than 0.27 but normal free thyroxine levels when patients were not on thyroid-altering medications.

Results

A total of 501 presentations, including 110 patients, were recorded over a period of 12 months. Mean age was 72 ±11 years (range 41-94 years). Some 21 patients were excluded as their clinical notes could not be found. Of the remaining 89 patients, there were 49 males and 40 females. Mean length of stay was three days (R 1-16 days). Thyroid function status was assessed in 87% (n = 77) of AF patients with mean TSH of 3.08mIU/L (R 0.22 to 30.5mIU/L) (see Figure 1). 

Figure 1. Assessment of thyroid function tests in patients with atrial fibrillation in St Luke’s Hospital(click to enlarge)

Some 23.2% (n = 16) of the population had thyroid dysfunction, of which 5.9% (n = 1) had overt hyperthyroidism, 35.3% (n = 6) had subclinical hypothyroidism and 58.8% (n = 10) of all the patients who had thyroid dysfunction had under-reactive thyroid (see Figure 2). 

Figure 2. Thyroid function assessment and intervention(click to enlarge)

Figure 4 summarises patients’ co-morbidities: 

69.7% (n = 62) of patients were hypertensive

29.2% (n = 26) had coronary arterial disease

27.0%(n = 24) had congestive heart failure

24.7% (n = 22) had ischaemic heart disease

16.9% (n = 15) had diabetes

11.2% (n = 10) had lower tract infection

7.8% (n = 7) had cerebrovascular accident or transient ischaemic attack.

Subgroup analysis

The team also assessed the services’ vigilance in assessing thyroid dysfunction in patients who had AF and also had cerebrovascular accident (stroke or transient ischaemic attack). Seven patients had either stroke or transient ischaemic attack. Of the 53% (n = 4 of 7), half of them had thyroid dysfunction and were on treatment (see Figure 3).

Figure 3. Assessment for thyroid dysfunction in stroke patients(click to enlarge)

Figure 4. Patient co-morbidities(click to enlarge)

Discussion

Thyroid dysfunction had been associated with increased risk of AF, atherosclerosis, cardiomyopathy (heart failure, mostly with preserved left ventricle function), ischaemic heart disease and increased mortality.^5,10,11 Both AF and thyroid dysfunction tends to increase with age,¹² therefore, affected patients tend to have more cardiovascular complications and increased mortality. Routine monitoring of thyroid function status by physicians in patients with AF may further play a major role in preventing complications caused by thyroid dysfunction.

In our audit the prevalence of thyroid dysfunction test abnormalities in patients admitted with known or new diagnosis of atrial fibrillation was 23.2% or almost one in four. This is a significantly high number compared to other studies.^13,14 However, this is not a surprise because other surveys were performed in the community while our study involves acute hospital admissions; therefore, most of the patients will be acutely unwell, resulting in abnormal thyroid function as a response to stressful body environment.^15,16 Another point to consider is that our population is older and thyroid dysfunction is expected to be higher with increased age.¹²

Our audit also highlighted deficiency in assessment for thyroid dysfunction in patients with AF and also in patients presenting with stroke or who had been diagnosed with stroke. This is despite the increased evidence associating thyroid dysfunction with atherosclerosis leading to cardiovascular disease, for example stroke, myocardial infarct and heart failure.⁵ Most of our patients are already at risk of cardioembolic stroke (high CHA2DS2-VASc scores) as they are older and about 70% are hypertensive.

Conclusion 

The prevalence of thyroid dysfunction in patients who were admitted with known or new atrial fibrillation was high. We recommend thyroid function assessment in all patients with AF to avoid complications associated with the disease.  

References

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