The effect of L-thyroxine replacement on blood pressure and heart rate in children with hypothyroidism: correlation with free thyroxine and thyrotropin levels

Introduction Thyroid hormones have direct and indirect cellular effects on the cardiovascular system. Chronic administration of L-thyroxine (LT4) to children with hypothyroidism may affect cardiovascular dynamics. Aim This cross-sectional observational study measured the blood pressure (BP) and heart rate (HR) of 25 randomly selected children with hypothyroidism in relation to their serum-free thyroxine (FT4), thyroid-stimulating hormone (TSH) levels, and thyroxine dose μg/kg. Patients and methods In total, 25 randomly selected children with a mean age 9.6±4.5 years with the diagnosis of congenital or acquired hypothyroidism. They were on l-thyroxine therapy to keep serum-FT4 and TSH in the normal range for age. Their BP and HR (average of three readings), weight, height, dose of thyroxine, and their serum level of TSH and free T4 levels within 2 weeks before the clinic visit were recorded. The Z scores of the height, height standard deviation score (HSDS), body mass index standard deviation score (BMISDS), systolic (SBPSDS), and diastolic blood pressure standard deviation score (DBPSDS) were calculated (as per the 2004 guideline report on hypertension from the National Heart and Lung Institute), and HR [as per the Monitoring and Diagnosis Group at Oxford (MADOX) systematic review of 2011] was calculated for age and sex. Results The authors studied 25 children with hypothyroidism. Their dose of thyroxine was 2.11±0.7 μg/kg. Their mean serum-FT4 levels were 15.1±2.5 μg/l and TSH level was 3.9±2.8 mIU/ml. Their HtSDS (−0.57±1.4) and BMISDS (0.63±1.63) were within normal. All had normal diastolic blood pressure readings (within 2 standard deviations of the mean for age and sex). Two of the individuals had systolic BPs greater than 2SDS above the mean for age and sex, with normal BMISDS. Both had normal FT4 and TSH levels. One patient had tachycardia [HR=146/min (Z-score +3.2 for age)]. His FT4 and TSH levels were normal. There was a positive correlation between thyroxine dose/kg and serum level of FT4 (r=0.52), HR (r=0.56), SBPSDS (r=0.38), and DBPSDS (r=0.24). There was no significant correlation between the HtSDS on the one hand and FT4, TSH, or thyroxine dose on the other hand. The BMISDS was negatively correlated with TSH level (r=−0.4). Conclusion In this study on hypothyroid children on l-thyroxine therapy, 2 had high systolic BP and one had tachycardia. The significant correlation between the thyroxine dose, FT4 level, and BP and HR should alert the pediatrician to monitor BP and HR closely in relation to TSH and FT4 levels of these patients.