Thyroid function tests and resting heart rate in Graves' disease: a prospective longitudinal study

Increased resting heart rate (RHR) is a typical sign of thyrotoxicosis. RHR may therefore serve as a monitoring tool, but longitudinal data on continuous measures of RHR and thyroid hormones are sparse. We evaluated the association between continuously monitored RHR and thyroid function tests in Graves' disease (GD).

This is a double-center, prospective, longitudinal cohort study in 30 patients with GD on antithyroid drug treatment. Participants were followed up with monthly study visits over six months at two hospitals in Graz, Austria.

Free thyroxine (fT4) and free triiodothyronine (fT3) were measured at each study visit. Participants were encouraged to continuously wear a wristband activity tracker (Fitbit Charge 6) to assess RHR. Associations of RHR (mean value over ten days) with thyroid hormones were determined by generalized estimating equation analyses.

From September 2024 to November 2025, we enrolled 30 participants (24 women, mean (SD) age of 40.7 (14.6) years) and performed 176 study visits. Thyrotoxicosis was present in 24 participants (80%) at baseline. One SD increase in RHR of 8.8 beats per minute was associated with fT4 (beta = 0.856, 95% CI: 0.641-1.071, P < 0.001), fT3 (beta = 0.836, 95% CI: 0.620-1.051, P < 0.001), and thyrotoxicosis (odds ratio: 5.134, 95% CI: 2.192-12.025, P < 0.001), respectively.

RHR assessed by a wearable device is strongly associated with thyroid hormones in GD. Further trials are required to evaluate whether RHR is useful to guide timing of thyroid function tests during antithyroid drug treatment and for GD diagnostics.