Comparative Evaluation of Thyroid Profiles and Heart Rate Variability in Newly Diagnosed Subclinical Hypothyroid and Euthyroid Pregnant Women

Background Subclinical hypothyroidism (SCH) is a common pregnancy condition that is frequently regarded as a biochemical finding with few functional implications. Thyroid hormones, however, have an impact on autonomic modulation, and even slight insufficiencies can change a mother's cardiovascular flexibility. Although heart rate variability (HRV) is a sensitive indicator of autonomic balance, there is little information available on HRV in pregnant women with SCH. Methods An analytical observational study was carried out on 41 pregnant women with recently diagnosed SCH and 40 pregnant controls who were euthyroid and in their second trimester. Lead II configuration resting five-minute ECGs were captured. LabChart Pro v8.1.30 software (AD Instruments, Castle Hill, Australia) was used to analyze them for HRV. Measurements were made in the frequency domain (low frequency (LF), high frequency (HF), LF/HF ratio, and total power), time domain (average duration of all normal-to-normal (NN) intervals between consecutive heartbeats (mean RR), standard deviation of normal-to-normal intervals (SDNN), root mean square of the standard deviation of interbeat intervals (RMSSD), and percentage of interbeat intervals varying by more than 50 ms (pNN50)), and nonlinear indices (standard deviation of the instantaneous beat-to-beat variability (SD1), standard deviation of the continuous long-term variability of RR intervals (SD2), and index of the balance between short-term and long-term variability (SD1/SD2)). With significance set at p<0.05, statistical analysis was conducted using either the Mann-Whitney U test or the unpaired t-test, depending on the distribution of the data. Results Mean RR was comparable between groups (p=0.331). Time-domain parameters were significantly lower in SCH: SDNN (p=0.005), RMSSD (p=0.035), and pNN50 (p=0.026). In the frequency domain, LF was significantly higher in SCH (p=0.04), whereas HF (p=0.281) and LF/HF ratio (p=0.143) were comparable. Total power was significantly reduced in SCH (p=0.0001). Nonlinear indices also showed reductions in SD1 (p=0.009) and SD2 (p=0.0001), while SD1/SD2 ratio remained comparable (p=0.585). Correlation analysis revealed no significant association between thyroid-stimulating hormone (TSH) and HRV indices (all p>0.05). Conclusion Subclinical hypothyroidism in pregnancy, though biochemically mild, is associated with early autonomic dysfunction characterized by reduced parasympathetic activity and diminished global HRV. These changes occur even in the absence of alterations in basal heart rate, suggesting subtle but functionally relevant cardiovascular impairment. HRV analysis may serve as a sensitive non-invasive tool for detecting early autonomic dysregulation in SCH and highlights the need for closer maternal surveillance during pregnancy.