Thyroid Dysfunction and the Autonomic Nervous System-Studies on Variations in R-R Intervals on the Electrocardiogram (Ecg) -

Little is known about the relation between thyroid dysfunction and the autonomic nervous system, particularly the parasympathetic nervous system. Variations in the R-R interval on the electrocardiogram (ECG) have been considered a reliable indicator of the parasympathetic nervous system abnormalities. We have attempted to apply this technique to patients with thyroid dysfunction.Studies were conducted in 60 healthy subjects, 60 patients with Graves' disease and 34 patients with Hashimoto's thyroiditis. Variations in R-R intervals were expressed as a coefficients of variance of 100 heart rates while resting (CVq) and deep respiration (CVd), respectively.A negative correlation between CVq and age was observed in control subjects. A similar phenomenon was also observed between CVd and age. CVq was significantly lower in untreated patients with Graves' disease than in treated, euthyroid patients with Graves' disease, and control subjects. Similar results were also observed with respect to CVd. CVq, on the other hand, was markedly decreased by administration of adrenaline and atropine in healthy subjects. Decreased CVq in untreated patients with Graves' disease was restored by antithyroid drug therapy and also by administration of propranolol and metoprolol, but not bunazosin. Patients with severe hypothyroidism (low T4, low T3 and elevated TSH) due to Hashimoto's thyroiditis had significantly lower CVq and CVd values than control subjects. Decreased CVq and CVd were restored by L-T4 therapy. Similar results were also found in the hypothyroid state during antithyroid drug therapy for patients with Graves' disease.The present investigation suggests that there is increased sympathetic activity associated with adrenergic β- (especially β1-) effects in patients with hyperthyroidism. Furthermore, a decrease in serum T3, in addition to a decrease in serum T4, may be required for functional abnormalities of the parasympathetic nervous system to develop in patients with hypothyroidism.