Differences in heart rate profile during exercise among subjects with subclinical thyroid disease

Background: Clinical thyroid disease is associated with changes in the cardiovascular system, including changes in heart rate during exercise. However, data on the relation between subclinical thyroid disease (SCTD) and heart rate during exercise are limited. Methods: We investigated 3799 apparently healthy subjects who were evaluated in the Institute for Preventive Medicine at the Sheba Medical Center. All subjects answered standard health questionnaires; were examined by a physician; completed routine blood tests including thyrotropin, free triiodothyronine, and free thyroxine levels; and underwent a treadmill exercise according to the Bruce protocol. Subjects with known thyroid disease or those who were taking thyroid-related drugs were excluded from the analysis. Heart rate profile was compared between patients with subclinical hypothyroidism (SCHypoT), patients with normal thyroid function, and patients with subclinical hyperthyroidism (SCHyperT) using propensity score matching. Results: Seventy patients had SCHyperT and 273 had SCHypoT. Compared with age- and sex-matched normal subjects, SCHyperT subjects had a higher resting heart rate (83±17 vs. 76±12 beats per minute [bpm], p=0.006), a significantly higher recovery heart rate (94±12 vs. 90±12 bpm, p=0.045), and a significantly lower heart rate reserve (80±20 vs. 87±18 bpm, p=0.006). Subjects with SCHypoT showed a trend toward a lower resting heart rate (75±13 vs. 77±15 bpm, p=0.09) and had a significantly lower recovery heart rate (88±12 vs. 90±13 bpm, p=0.035). There was no significant difference in exercise duration or blood pressure between subjects with SCTD and their matched normal controls. Conclusions: Subjects with SCTD have a significantly different heart rate profile during rest, exercise, and recovery.