Relationship between cardiac arrhythmias and subclinical hyperthyroidism
P De Remigis1, L Vianale1, A De Remigis1, P Sabatini2 & E Di Girolamo2
Atrial fibrillation (AF) and, in general, cardiac arrhythmias are among the most frequent complications during clinical thyrotoxicosis. However to-day the most frequent conditions of hyperthyroidism are subclinical and more subtle is the relationship between this last condition and cardiac arrhythmias.
In order to evaluate this topic, we considered two groups: A) patients admitted into the hospital for AF; B) patients affected by subclinical hyperthyroidism (SH) (low TSH, less than 0.1mUI/ml, normal FT4 and FT3). Patients of group A were evaluated for thyroid dysfunction and those of group B for cardiac rhythm changes by ECG-Holter.
A condition of SH, unknown before AF episode, that brought to the hospital admission, was diagnosed in 10% patients of group A, while a 4% showed a clinical hyperthyroidism. In the follow-up the treatment of subclinical hyperthyroidism has determined a prophylaxis of FA. In the group B we did not observed any case of FA, but only an increased mean Cardiac Frequency (FCm) (Table):
FA is complex phenomenon from pathogenetic point of view, not well explained. It is the concourse of many factors, grouped in three classes: trigger, cardiac and extracardiac factors, unbalance of vagal and simphatetic tone. These observations suggest on one hand subclinical hyperthyroidism is per sè a determinant cause of FA, as the result in the first group could demonstrate; in the other hand it is a factor that necessitate of others to precipitate cardiac arrhythmias, as the result of second group could suggest.
Another explanation is in peripheral modulation of thyroid hormones (TH) at nuclear receptors of miocites, that can respond in different way to acute or chronic exposition of TH.