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Plasma free fatty acids in neonates with congenital hypothyroidism
1The Department of Pediatrics, School of Medicine, Niigata University, Japan. asamitds@medws1.med.niigata-u.ac.jp
Insights
Thyroid hormones do not significantly impact fat mobilization in infants with congenital hypothyroidism (CH). This finding helps explain why even severely affected newborns show mild symptoms, despite screening detecting numerous CH cases.
Area of Science:
- Pediatrics
- Endocrinology
- Neonatal Medicine
Background:
- Neonatal screening detects many congenital hypothyroidism (CH) cases.
- Severely hypothyroid neonates often lack typical symptoms like bradycardia.
- Limited data exist on thyroid hormone effects on energy expenditure in neonates.
Purpose of the Study:
- Investigate fuel utilization in neonates with CH.
- Assess the role of thyroid hormones in lipolysis and energy expenditure.
- Compare plasma free fatty acids (FFAs) before and after levothyroxine (LT4) therapy.
Main Methods:
- Enrolled 55 screen-detected CH neonates and 29 controls.
- Divided CH neonates into mild (TSH <100, FT4 ≥0.6) and severe (TSH ≥100, FT4 <0.6) groups.
- Measured heart rates, plasma FFAs, and thyroid hormones (TSH, FT4) before and after LT4 therapy.
Main Results:
- No significant differences in plasma FFA levels among mild CH, severe CH, and control groups before therapy.
- Plasma FFA levels significantly increased in both CH groups after LT4 therapy.
- No significant differences in plasma FFA levels among groups at 3 months; no correlation found between FFAs and thyroid hormones.
Conclusions:
- Thyroid hormones may not play a major role in fat mobilization via adrenergic lipolysis in neonates and young infants.
- This finding may explain the mild clinical presentation of screen-detected CH neonates.
- Further research is needed to fully understand thyroid hormone's role in neonatal energy metabolism.
Abstract:
Since the introduction of neonatal mass screening for congenital hypothyroidism (CH), numerous cases have been detected. It is of interest that even severely hypothyroid neonates rarely exhibit bradycardia, hypothermia, or inactivity, which have been recognized as typical signs of CH. Regarding neonates and young infants, few reported data are available on the effects of thyroid hormones on energy expenditure. Plasma free fatty acids (FFAs), markers for lipolysis, play essential roles in maintaining physiologic homeostasis. To study fuel utilization in CH neonates, we measured heart rates, plasma FFA, and thyroid hormones before and after levothryoxine (LT4) replacement therapy. Fifty-five screen-detected CH neonates and 29 age-matched normal neonates for controls were enrolled. The CH neonates were divided into two groups according to serum thyroid hormone levels: a mildly hypothyroid group (n = 37), serum thyrotropin (TSH) less than 100 microIU/mL and free thyroxine (FT4) 0.6 ng/dL or more; and a severely hypothyroid group (n = 18), TSH 100 microIU/mL or more and FT4 less than 0.6 ng/dL. Twenty-four of the 55 patients had their heart rates measured by electrocardiography. Fasting blood samples were taken from the subjects during physical movements. Serum levels of TSH, FT4, FFA, and other blood chemicals, measured on an autoanalyzer system in our hospital, were compared before and after LT4 substitution therapy. The following results were obtained. The mean plasma FFA values before LT4 replacement were 208.5 +/- 89.4 microEq/L in the mildly hypothyroid group, 228.5 +/- 114.7 microEq/L in the severely hypothyroid group, and 213.9 +/- 97.7 microEq/L in controls. No statistical differences were noted among the three values. Two months after LT4 replacement therapy, at the age of 3 months, plasma FFA concentrations significantly increased in both groups compared with those before the therapy. Control infants also showed a significant increase in plasma FFA concentrations from 1 to 3 months of age. There were no significant differences in plasma FFA concentrations among the three groups at the age of 3 months. No significant correlations were found between plasma FFA and serum thyroid hormones. From these results it is suggested that in neonates and young infants, thyroid hormones do not play major roles in mobilization of fats through the adrenergic regulation of lipolysis for energy supply. This may be one of the reasons for the unexpectedly mild signs and symptoms in the screen-detected hypothyroid neonates.