Impaired sensitivity to thyroid hormones is associated with lower heart rate in the euthyroid population.

Background: The association of thyroid hormone sensitivity with heart rate remains unclear.

Objective: This study aims to elucidate the relationship between impaired thyroid hormone sensitivity and lower heart rate in the euthyroid population.

Methods: A total of 550 participants were included. Heart rate and serum biochemicals were measured. Thyroid hormone sensitivity indices were calculated using the thyrotropin index (TSHI), thyrotropin thyroxine resistance index (TT4RI), thyroid feedback quantile-based index (TFQI), Chinese-referenced parametric TFQI (PTFQI), and the ratio of free triiodothyronine to free thyroxine (FT3/FT4 ratio). Logistic regression analyses were applied to explore the relationship between thyroid hormone sensitivity indices and heart rate.

Results: TSHI, TT4RI, TFQI, and PTFQI were higher, and FT3/FT4 ratio was lower in participants with heart rate ≤ 60 beats/min (P < .001 for all). Participants with increased TSHI, TT4RI, TFQI, and PTFQI and reduced FT3/FT4 ratio had lower heart rate (≤60 beats/min) (P for trend < .001 for all). Odds ratios (95% confidence intervals) for TSHI, TT4RI, TFQI, PTFQI, and FT3/FT4 ratio in the highest quartile were 2.090 (1.092-4.000), 2.240 (1.151-4.361), 2.014 (1.043-3.887), 2.163 (1.123-4.166), and 0.498, (0.249-0.996), respectively, as compared with the lowest quartile after adjusting for sex, age, body mass index, smoking, drinking, hypertension, diabetes, coronary artery disease, glycated hemoglobin, total cholesterol, low-density lipoprotein cholesterol, and triglycerides.

Conclusions: Impaired sensitivity to thyroid hormones was associated with lower heart rate in euthyroid participants. Future large-scale studies are needed to confirm our findings.