Plasma Free Fatty Acids in Neonates with Congenital Hypothyroidism

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 bardycardia, 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 μIU/mL and free thyroxine (FT₄) 0.6 ng/dL or more; and a severely hypothyroid group (n = 18), TSH 100 μIU/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 μEq/L in the mildly hypothyroid group, 228.5 ± 114.7 μEq/L in the severely hypothyroid group, and 213.9 ± 97.7 μEq/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.