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From a clinical viewpoint, the most important electrocardiographic abnormality
in H is AF. AF, usually persistent rather than paroxysmal, develops in
10-28% of cases with H. It is significantly more common in older patients,
reflecting a reduction in the threshold for AF with age, later diagnosis
and an increase in the prevalence of coexistent ischaemic and degenerative
heart disease (69). In one series, 25% of patients with H >60yr had AF
compared with a 5% prevalence of AF in patients <60yr (70). Patients
with toxic goitre also showed an increased prevalence of AF (43%) vs 10%
only in younger patients with Graves' disease (71). Analysis of rhythm
disorders in 219 patients with H (72) showed an age-dependent distribution
of AF and sinus node dysfunctions. In order to study the relationship between
the left atrial size and AF in H, 92 patients with Graves' disease were
examined (73). Nineteen (21%) had AF; 31% of the patients >40yr had AF
but none of those <40. Left atrial enlargement existed in 7% of patients
<40yr, in 2% of those >40 without AF and in 94% of those >40yr with
AF.
Low TSH is a risk factor for later development of AF (74). 2007 clinically euthyroid subjects who were >60 yr and in sinus rhythm were followed to determine the frequency of AF over the next 10 yr. The cumulative incidence of AF was 28% among subjects with low TSH (<0.1 mU/L) and 11% among 1576 subjects with normal values. Overt H (but not AF) subsequently developed in two people with low TSH and one with normal TSH. After adjustment for other known risk factors, the relative risk of AF in the subjects with low TSH was 3.1 (95% confidence interval 1.7 to 5.5). Two thirds of the low TSH subjects were being treated with T4, however, excluding these subjects has little effect on the relative risk of AF associated with low TSH. Mean T4 concentration was slightly higher in the low TSH group but was within the normal range in 84% of those not on T4 replacement and was not correlated with the subsequent occurrence of AF. Thus, regarding the high incidence of AF in older patients with H, it is important to detect subclinical H in this group. Once euthyroidism is restored, all patients who revert to sinus rhythm (about 60%) spontaneously do so within 4 months of being euthyroid (75). Besides age, the main determinant of reversion to sinus rhythm appears to be the duration of AF: patients who had been in AF for more than 1 yr and those who are older are likely to need intervention in the long run, probably reflecting the coexistence of intrinsic heart disease in these thyrotoxic AF patients. |
