Effects of Thyroid Hormone Deficiency on Electrocardiogram Findings of Congenitally Hypothyroid Neonates

Hypothyroid status is believed to cause various metabolic changes in infants. However, it is interesting that even severely hypothyroid neonates, detected during mass neonatal screening, rarely show bradycardia, hypothermia, or inactivity. To study cardiac functions of screen-detected neonates with congenital hypothyroidism (CH), we recorded the electrocardiograms (ECG) of 53 screen-detected CH neonates before levothyroxine (LT4) replacement therapy, and 15 age-matched normal neonates for controls. The 53 CH neonates were divided into two groups according to initial serum thyroid hormone levels: a mildly hypothyroid group (n= 37), serum thyroid-stimulating hormone (TSH) less than 100 μIU/mL and free thyroxine (FT4) 0.6 ng/dl or more; and a severely hypothyroid group (n= 16), TSH 100 μIU/mL or more and FT4 less than 0.6 ng/dL. TSH, FT4, and other blood chemicals were measured on an autoanalyzer (Hitachi 7170). After blood sampling, the ECG was recorded during induced sleep by oral administration of triclofos sodium syrup. ECG parameters, including HR, PR, QRS, QT time and corrected QT time (QTc) were automatically obtained, using an auto-ECG analyzing system. The following results were obtained. No CH patients showed abnormal ECG findings. There was no significant difference of the mean heart rates (HRs) between the mildly hypothyroid (147.5 ± 16.3 beats per minute) and the control group (148.3 ± 12.1 beats per minute). The mean HR in the severely hypothyroid group (134.0 ± 17.9 beats per minute, p= 0.007) was significantly low compared with the normal control group. However, all values were within normal ranges. QTc in the severely hypothyroid group (0.414 ± 0.015, p= 0.033) was significantly shorter than in the control group (0.440 ± 0.052). No statistical differences of PR, QRS, and QT time were noted among the three groups. All ECG parameters were within normal ranges. HR positively correlated with FT4and log (FT4), and negatively with TSH and log (TSH). From these results we conclude that the deficiency of thyroid hormones does not affect ECG findings of congenitally hypothyroid neonates. This may be consistent with the unexpectedly mild signs and symptoms of screen-detected hypothyroid neonates.