Plasma free fatty acids in neonates with congenital hypothyroidism.
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.
The Department of Pediatrics, School of Medicine, Niigata University, Japan. asamitds@medws1.med.niigata-u.ac.jp
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Asami
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Wada
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Uchiyama
D003409:Congenital Hypothyroidism; D004562:Electrocardiography; D004734:Energy Metabolism; D005230:Fatty Acids, Nonesterified / Q000097:blood*; D006339:Heart Rate; D020249:Hormone Replacement Therapy; D006801:Humans; D007037:Hypothyroidism / Q000097:blood*; D007231:Infant, Newborn; D008066:Lipolysis; D015997:Neonatal Screening; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood / Q000627:therapeutic use
D005230:Fatty Acids, Nonesterified; D013972:Thyrotropin; D013974:Thyroxine