Decreased HR in dogs with spontaneous and experimentally induced hypothyroidism has been reported,7, 9, 11, 12 and may be caused by a direct effect of thyroid hormones on the myocardium, decrease in tissue oxygen consumption, or downregulation of β‐adrenergic receptors causing a decreased response to sympathetic stimulation.35 The P‐ and R‐wave amplitudes also were significantly decreased in the hypothyroid group compared to those of the control group. Here, TG is linked to hypothyroidism.