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| Journal of the New Zealand Medical Association,
07-November-2008, Vol 121 No 1285
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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.
Method300 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.
ResultsPatients 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.
DiscussionTo 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
References:
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