Heart rate recovery and chronotropic incompetence in patients with subclinical hypothyroidism.

Background: Heart rate recovery (HRR) and chronotropic incompetence (CI) in patients with subclinical hypothyroidism (SCH) has not been explored previously. The aim of the present study was to evaluate the HRR and CI in patients with SCH.

Methods: Twenty-five patients (11 men, 14 women with a mean age of 36 + or - 10 years) who were diagnosed SCH determined by an increased serum thyrothrophine (TSH) concentration (>4.0 ng/mL) and the normal free triiodothyronine (fT3) and free thyroxin (fT4) levels, were included in the study. The control group of healthy individuals with normal TSH (12 males, 15 females) with a mean age of 36 + or - 3 years was also included. Two groups were well matched for age, sex, and body mass index. Medical history, physical examination, electrocardiogram, treadmill exercise testing, and chest radiogram were performed for all participants.

Results: The characteristics of SCH patients and control cases were similar with regard to age, sex, and BMI except for TSH levels. Serum TSH levels were significantly higher in SCH patients than the controls (P < 0.001). No significant differences were observed in the changes of heart rate (HR), exercise tolerance (metabolic equivalents), or systolic and diastolic blood pressures at rest or during exercise between the groups, whereas HRR and CI were significantly lower during exercise testing in the SCH patients compared to controls (P < 0.003; P < 0.03, respectively).

Conclusion: The results of the present study demonstrated that SCH can cause impaired cardiovascular autonomic function and attenuated HR response to exercise. (PACE 2010; 2-5).