Linear and nonlinear analyses of heart rate variability following orthostatism in subclinical hypothyroidism

Subclinical hypothyroidism (SCHypo) is associated with autonomic disturbances that can interfere in physiological responses. This study was designed to evaluate linear and nonlinear variables of heart rate variability (HRV) following postural change, comparing subjects with SCHypo to euthyroid subjects. HRV analyses were performed in 5-minute time series collected in the supine and standing positions from a subsample of 855 participants of the ELSA-Brasil study. The cardiac autonomic nervous function was evaluated by linear time and frequency domain analyses (SDNN, RMSSD, LFms(2), HFms(2), and LF/HF ratio) as well as by nonlinear symbolic dynamics (0, 1, and 2V). After exclusions, 509 (92.0%) euthyroid and 44 (8.0%) SCHypo participants were eligible for analyses. At the baseline supine rest measurement, the 0V symbolic pattern was higher (27.7 vs 25.4, P=.02) and 2V was lower (18.0 vs 22.9, P=.02) than in the euthyroid group. Comparing the variation between positions, the 0V pattern showed a lower delta in SCHypo than in Euthyroid subjects (8.0 vs 10.8%, P=.04). SCHypo presented lower sympathetic and parasympathetic tonus at rest and a blunted sympathetic response to active postural change, marked by reduced variation in the 0V of symbolic analysis (SA). Additionally, it is suggested that SA of HR dynamics is an alternative and, possibly, a more sensitive method for cardiac autonomic assessment following orthostatism in this population.