Correlation between function of the pituitary-thyroid axis and metabolism of catecholamines by the fetus at delivery.

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The relationship between the pituitary-thyroid axis and the metabolism of catecholamines has been studied in 33 maternal, arterial and venous cord blood specimens obtained at the time of normal vaginal delivery, following uncomplicated pregnancies. Ten fetal cord blood samples were collected at the time of elective caesarean section. Levels of noradrenaline and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured in all these samples, using the high performance liquid chromatography (HPLC)-electrochemical detection method, and TSH was measured by radioimmunoassay. In cord arterial blood samples at vaginal delivery the levels of plasma noradrenaline and serum TSH were significantly increased, but those of DOPAC, which reflects central dopaminergic activity, were unchanged when compared to samples obtained at elective caesarean section. Within the same samples there was a significant correlation between the levels of plasma noradrenaline and those of serum TSH. Noradrenaline is believed to have a stimulatory and dopamine an inhibitory influence on TSH secretion. The fetal thyroid gland seems to respond adequately to the stimulus of TSH at term. The present study suggests that catecholamines, especially noradrenaline, may regulate secretion of TSH from the pituitary gland in the fetus at delivery and that elevation of TSH probably induces a subsequent rapid increase in thyroid hormones in the neonate.