Studies suggest that patients with subclinical hypothyroidism (SH) have sympathovagal imbalance, which could lead to a slower heart rate (HR) response in the transition from rest to exercise. Thus the objective of this study was to investigate the behavior of the HR kinetics in patients with SH during the transition from rest to exercise. The study included 18 SH women [thyroid stimulating hormone (TSH) = 6.95 +/- 2.94 mu IU/ml and free thyroxine (FT4) = 0.96 +/- 0.15 ng/dl] and 17 euthyroid women (TSH = 2.28 +/- 0.84 mu IU/ml and FT4 = 0.98 +/- 0.07 ng/dl). Both groups were matched for physical activity, menopausal status, and age. The HR kinetics was obtained during the course of a constant-load exercise (50 W), for 6 min, in a cycle ergometer, and quantified from the mean response time (MRT), which is equivalent to the time taken to reach 63% of the HR at steady state. SH patients showed slower HR kinetics than the control group (MRT = 48.5 +/- 17.6 vs. 36.0 +/- 10.3 s, P = 0.015). The MRT has been shown to correlate with the level of physical activity (r = -0.361; P = 0.033) and with the subjective perception of exertion at the end of the exercise (r = 0.365; P = 0.031). It is concluded that SH patients have slower HR kinetics in the transition from rest to exercise compared with euthyroid women, with this impairment being associated with lower levels of physical activity. NEW & NOTEWORTHY Subclinical hypothyroidism patients have slower heart rate kinetics in the transition from rest to exercise when performing a constant-load exercise at 50 W.