Relationship of Thyroid Hormones with Heart Rate Variability

Background: Altered thyroid functions are associated with variation in autonomic regulation ofcardiovascular activity. Cardiac Autonomic Nervous Activity (CANA) can be assessed quantitativelyby analysis of Heart Rate Variability (HRV). Objective: To observe the relationship between CANAwith altered TSH and FT4. Methods: This cross sectional study was carried out in the Department ofPhysiology, BSMMU, Dhaka between1st July 2007 and 30th June 2008 on 60 patients with excessthyroid hormone (group B, aged 30-50 years). Based on treatment, 30 untreated newly diagnosedpatients were designated as group B1 and 30 patients under treatment with antithyroid drugs for atleast 2 months were included into group B2 in order to observe the effect of treatment. All thesepatients were selected from the Out Patient Department of Endocrinology wing of Department ofMedicine, BSMMU, Dhaka. Sociodemographically matched 20 apparently healthy euthyroid personswere selected for comparison (group A). To confirm thyroid status, serum TSH and serum FT4 levelswere measured by AxSym system and some of the spectral HRV parameters i.e.mean R-R interval,mean heart rate, variance, LF n.u, HF n.u and LF/HF ratio were assessed by recordings of ECG for 5minute (short term) with a polyrite. For statistical analysis Pearson’s correlation coefficient (r) test wasused. Results: With serum TSH level, the LF n.u. power and LF/HF ratio showed significant (p<0.05)positive correlations but HF n.u. power showed significant (p<0.05) negative correlation in group B1.But these three parameters showed non significant correlations with TSH in the other two groups (A,B2). Similarly with serum TSH level, variance and mean R-R interval showed negative and mean HRshowed positive correlation in group B1. In group A, all these parameters were positively correlatedwhereas in group B2, RR interval and variance were positively and mean HR was negatively correlated.All these correlations were statistically non significant. With serum FT4 levels, mean R-R and HF n.u.were negatively and mean heart rate, LFnu, LF/HF were positively correlated in all three groups butvariance showed positive in group A and negative correlation in B1 and B2. All these correlations werestatistically non significant. Conclusion: From this study it can be concluded that changes in autonomicnervous regulation are related to altered serum level of TSH and FT4 in hyperthyroids.Key words: HRV; LF; HF; RR interval; HyperthyroidDOI: 10.3329/jbsp.v5i1.5414J Bangladesh Soc Physiol. 2010 June; 5(1): 20-26