Parasympathetic nervous system activity in hypothyroidism determined by R-R interval variations on electrocardiogram

Little is known about the relationship between hypothyroidism and the parasympathetic nervous system. R-R interval variations revealed by electrocardiogram (ECG) are known to be a useful clinical indicator of abnormalities of parasympathetic nervous system activity. Studies were conducted in hypothyroid patients, and significant reductions in R-R interval variations were observed in patients with primary severe hypothyroidism due to Hashimoto's thyroiditis, and in patients with Graves' disease who became severely hypothyroid during antithyroid drug therapy. R-R interval variations were restored to normal levels in both groups of patients after treatment. The present investigation suggests that in marked hypothyroidism there are hypofunctional abnormalities in the parasympathetic nervous system in association with a reduction in the levels of serum T4 and T3.