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Background: Hypothyroidism is an endocrine disease that leads to a reduction in the hormones thyroxine (T3) and triiodothyronine (T4), which therapy with levothyroxine restores the clinical signs related to the metabolic rate. Due to the influence of thyroid hormones on the heart, which is under the constant influence of the autonomic nervous system (ANS), dogs with hypothyroidism can develop bradycardia, arrhythmia, and dysautonomia.  Heart rate variability (HRV) assesses autonomic modulation by the Holter method, which is scarce in dogs. We aimed to report the cardiac and autonomic effects of the primary hypothyroidism untreated and treated with levothyroxine in a canine case by Holter monitoring.Case: A 7-year-old female Dalmatian, weighing 36 kg, was referred for clinical evaluation due to apathy, weight gain, low hair quality, and lethargy. On physical examination, alopecic lesions on the hind limbs and tail, as well as bradycardia with a heart rate (HR) of 40-50 beats per minute (bpm) were observed, in addition to a 3/6 mitral murmur and 2/6 tricuspid murmur. Given the suspicion of thyroid gland disorder, the blood hormonal measurement revealed an increase in thyroid-stimulating hormone (TSH; 0.65 ng/mL) and a decrease in free T4 (0.11 ng/mL) and total T4 (0.44 ng/mL), confirming primary hypothyroidism. Therapy was started with a minimum dose of levothyroxine (0.913 mg, every 12 h), which clinical signs were restored in five months of treatment, with weight loss, hair growth, and active behavior. To assess the impact of untreated and treated hypothyroidism on the patient’s ANS, a Holter monitoring exam was performed for 24 h before and after therapy. Before treatment, the average HR was 75 bpm, and the HR 2.0s), 1st-degree atrioventricular blocks (AVB), six ventricular ectopias events, and 2nd-degree sinoatrial block (SAB) were also observed. The ANS parasympathetic tone was significantly stimulated, highlighting bradycardia, arrhythmia, and dysautonomia. After five months of treatment with levothyroxine, the average HR was 89 bpm, and the HR&lt;50 bpm occurred during 02 h 06 min 13 s. No ventricular pauses, blocks, or ectopias were observed, showing the stimulation of sympathetic tonus, which restored HR and ANS balance. Still, it was observed that the minimum levothyroxine dose corrected cardiac changes by increasing the low frequency (LF), decreasing the high frequency (HF), and, consequently, increasing the LF/HF ratio, normalizing the frequency conditions in HRV.Discussion: In the frequency index, HF indicates the vagal activity, whereas LF indicates both systems with parasympathetic predominance. Before treatment, the dog had a low LF/HF ratio (0.46), indicating dysautonomia with parasympathetic stimulation. After therapy, the conditions of bradycardia and functional cardiac capacity were corrected, restoring ANS, due to the serum recovery of thyroid hormones. This study reported the cardiac and autonomic effects of primary hypothyroidism untreated and treated with levothyroxine on a dog, that had intense bradycardia and abnormal stimulation of the parasympathetic tone, associated with episodes of 1st-degree AVB, ventricular ectopias, and 2nd-degree SAB. After therapy with a minimum dose of levothyroxine, there was a decrease in parasympathetic activity and an increase in sympathetic stimulus, correcting cardiac changes, and restoring the balance of ANS. As it is a simple, non-invasive, and safe tool that helps the clinician to understand cardiac autonomic modulation, it is recommended to adopt the Holter monitoring exam in cases of hypothyroidism cases to assess sympathetic-vagal balance and check potential cardiac risks.

Minimum Dose of Levothyroxine Restored the Autonomic Balance on a Dalmatian Female Dog with Primary Hypothyroidism