Journal of the New Zealand Medical Association, 07-November-2008, Vol 121 No 1285
A survey of thyroid function test abnormalities in patients presenting with atrial fibrillation and flutter to a New Zealand district hospital
David D W Kim, Simon Young, Rick Cutfield
Hyperthyroidism is a known risk factor for atrial fibrillation and flutter (AF),1–3 and can aggravate ischaemic heart disease and heart failure.2,4 It is therefore generally recommended that thyroid function tests (TFT) be checked on a routine basis in patients with new onset AF, poorly rate controlled AF and recurrences of AF,5,6 since abnormalities are relatively easy to treat and may help control and stabilise cardiac disease. However, it is not clear how commonly AF is related to thyrotoxicosis in people presenting to hospital.
Amiodarone, a frequently used drug in the treatment of AF, can have direct toxic effects on thyroid cells and has high iodine content that can interfere with normal thyroid function. Both overactive and under-active thyroid dysfunction have been well described with use of amiodarone.7,8 Therefore, when amiodarone is being considered, it is recommended to measure TFT before and during its use.9
Iodine-based radiocontrast similarly can cause thyroid dysfunction and the prevalence of its recent use in patients with AF is not well described.
This audit was conducted in patients with AF presenting to North Shore Hospital (NSH) in Auckland, New Zealand, to evaluate how often TFT were requested, the prevalence and subtype of thyroid disease, adequacy of management of thyroid dysfunction, and the possible relationship to the recent use of amiodarone and radiocontrast.
300 consecutive patients were selected from the NSH AF registry. The registry was compiled in 2006 by the NSH Cardiology department for a study to review AF prevalence and management. Patients were retrospectively collected by screening discharge coding. These patients had been admitted to NSH between August 2005 and March 2006 with a diagnosis of atrial fibrillation or flutter. Of the 300, data of 250 patients who had ‘active management issue of AF’ during the encounter admission was included in the analysis. Active management issue of AF was defined as either; AF being the reason for admission; or a rate control issue during the admission; or a paroxysm or an episode of AF during the admission. The 50 who were excluded had chronic AF with adequate rate control, hence no routine indication for TFT.
Patients’ computerised clinical records were retrospectively reviewed and analysed. The data collected include patient’s age, gender, ethnicity, known thyroidal illness prior to admission, presence of TFT results, the actual TFT results, and concurrent use of amiodarone. If TFT were abnormal, the presence of thyroid auto-antibody results was sought, as well as records of recent (<3 months) radiocontrast use, and whether appropriate management of the abnormal TFT was undertaken (including a referral to or advice from endocrinology service, appropriate change in thyroid drug therapy, or a plan for appropriate follow up laboratory monitoring). The data analysed was that collected at the time of the admission from which the patient was enrolled into the AF registry.
Thyroid function results were interpreted and categorised into five groups. Subclinical hyperthyroidism if suppressed TSH <0.10 mU/L with normal T4 and T3; overt hyperthyroidism if suppressed TSH <0.10 mU/L with elevated T4 and/or T3; subclinical hypothyroidism if TSH >4.0 mU/L with normal free T4 (and T3 if performed); overt hypothyroidism if TSH >4.0 mU/L with depressed T4; and euthyroid if none of the above.
Finally, prevalence of thyroid dysfunction in our selected population was compared to other population survey data, and Chi-squared test was used to evaluate the statistical differences in the prevalence.
Patients with AF had a mean age of 71 years (22 to 93 years) with a 50% male/female split. Ethnic breakdown showed a predominance of Caucasians with 62% being New Zealand European, 26% other European, 6.8% Maori/Pacific Islanders, 1.6% Asian, and 2.4% other ethnicity. This likely represents population characteristics of NSH catchment area for this age group.
Eighteen of 250 patients (7.2%) had a known background diagnosis of thyroid dysfunction prior to the admission. Sixteen of them had hypothyroidism on thyroxine replacement.
23% of patients had not had TFT recorded either during the encounter admission, or in the preceding 6 months. Of those who had thyroid function measured, 90% had them taken at the time of admission.
Figure 1. Types and percentage of thyroid function test abnormalities
Of the 192 patients who had TFT measured, 34 (18%) had some abnormality. Subclinical hyperthyroidism was noted in 4 (2.1%) and overt hyperthyroidism in 6 (3.1%). Of these 10 patients (5.2%), mean age was 67 years, male to female ratio was 1:2.3. Seven were newly discovered, while three had known hypothyroidism on thyroxine replacement. Subclinical hypothyroidism was noted in 21 (11%) with overt hypothyroidism in three (1.6%). Two of the 21 with subclinical hypothyroidism, and one of the three with overt hypothyroidism, had previously known hypothyroidism on thyroxine treatment.
Of the 34 patients with abnormal TFT, 2 (5.9%) had thyroid antibody checked, and 10 (29%) had appropriate management as defined in methods above.
Of the 250 AF patients, 12 (4.8%) were on amiodarone at the time of presentation. Of these, 11 (92%) had TFT checked during the encounter admission or prior 6 months. One patient was diagnosed with amiodarone induced hyperthyroidism, and this was thought to have contributed to poor rate control of AF. One of the 12 patients on amiodarone had known hypothyroidism and was biochemically euthyroid on thyroxine replacement.
Seven (21%) of the 34 patients with abnormal TFT had received recent (<3 months) radiocontrast. The majority were related to computed tomography (CT) scans. Of these 7 patients, 6 had subclinical hypothyroidism and one had subclinical hyperthyroidism.
To our knowledge, this is the first published data describing the prevalence of thyroid dysfunction in patients with AF presenting to a general hospital in New Zealand.
The rates of newly diagnosed subclinical (2.1%) and overt hyperthyroidism (3.1%) in our study were higher than one would expect from general population prevalence surveys. These general population surveys,1,10–12 which looked at prevalence of thyroid dysfunction in community dwelling subjects of similar age group as our study, have shown prevalence of overt hyperthyroidism of 0.2–0.6% and subclinical hyperthyroidism of 1.3–2.1%. However, these studies did not look specifically at a population of patients hospitalised with AF like ours. The higher prevalence of overt hyperthyroidism was not surprising given the known effects of hyperthyroidism upon the cardiovascular system.
Nevertheless, as a possible contributing cause of AF in hospital patients, hyperthyroidism, overt or subclinical, was not present in 95%, and even in the 5% in whom it was found, it may not have been the only cause. We have no details on other possible contributing factors to the development of AF in these patients.
The significantly higher rate of subclinical hypothyroidism (11%), could have been a transient phenomenon in some cases in the context of non-thyroidal illness. However, it is of interest that a similar prevalence (15%) was found by Cappola et al1 in a community survey of 3233 over 65 year olds in the United States. Our rate of overt hypothyroidism was 1.6%, slightly higher than that found in the community prevalence studies mentioned above.
In our survey, most patients with abnormal TFT were not followed up appropriately. While only a minority needed referral in some way to the endocrinology service, it is reasonable to expect at least arrangements for follow-up TFT, and many did not have this. Thyroid antibodies were rarely checked. While their measurement might not add much to clinical management, their presence helps confirm possible autoimmune thyroid disease and can help guide follow up, especially in those with subclinical hypo and hyperthyroidism.
Amiodarone was not commonly implicated as a cause of abnormal TFT in our study. Only 12 patients (4.8%) were on amiodarone and only one had overt hyperthyroidism. While recent exposure to radiocontrast was common (21%) in subjects with abnormal TFT, clinical relevance of this was not totally clear.
Our data shows that we would need to screen nearly 20 people to detect one case of either overt or subclinical hyperthyroidism in patients presenting with AF. This would cost about NZ$200 if both TSH and T4 are used for screening.
Checking for hyperthyroidism in patients with AF is clinically important. While there is controversy over the management of subclincal hyperthyroidism, there is less disagreement in the setting of atrial fibrillation. Management of overt hyperthyroidism is uncontroversial in the setting of AF especially in the elderly.
In summary, our survey of a district general hospital showed that 5.2% of patients presenting with AF had clinical or subclinical hyperthyroidism. There was a significant prevalence of other types of thyroid dysfunction. It seems reasonable to continue to recommend TFT screening in patients presenting with AF.
Competing interests: None known.
Author information: David D W Kim, Endocrinology Registrar; Simon Young, Consultant Endocrinologist; Rick Cutfield, Clinical Director of Endocrinology; Department of Endocrinology, North Shore Hospital, North Shore, Auckland
Correspondence: David Kim, Endocrinology Registrar, Diabetes and Endocrinology Services, North Shore Hospital, Shakespeare Road, Takapuna, Auckland, New Zealand. Fax: +64 (0)9 4418986; email: David.Kim@waitematadhb.govt.nz
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