Immunoreactive thyrotrophin-releasing hormone like material (IR-TRH LM) in human cord blood: high circulating serum levels but possible non-identity with hypothalamic TRH

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Abstract
Levels of immunoreactive TRH-like material (IR-TRH LM) were measured in paired maternal and umbilical cord serum samples (n = 45) and in serum of non-pregnant women (n = 63), using a sensitive and specific radioimmunoassay. In all pairs IR-TRH LM in venous cord serum (mean 91.3 pg/ml, range 20-270) was markedly elevated as compared to maternal serum (mean 13.5 pg/ml, range 0-37, P < 0.001, the maternal levels being similar to those in non-pregnant women (mean 12.0 pg/ml, range 0-39, P > 0.1). In 23 cases IR-TRH LM was also measured in arterial cord serum: arterial and venous cord levels were highly significantly correlated (r = 0.904) with higher arterial levels in 22 of 23 cases (mean 117.8 pg/ml, range 32-280 and 90.6 pg/ml, range 20-270, P < 0.001), suggesting that cord IR-TRH LM is not of placental origin. There was no correlation between individual levels of IR-TRH LM and either TSH, T4 [thyroxine], T3 [triiodothyronine] or F[free]T4 levels in either maternal or cord serum. Agar gel electrophoresis and equilibrium dialysis of adult and cord serum pre-incubated with [125I]TRH or [3H]TRH revealed no protein binding. Cord IR-TRH LM was immunologically, as well as in paper electrophoresis and in gelfiltration, indistinguishable from synthetic and hypothalamic TRH. In vitro degradation of synthetic TRH was much slower in cord serum as compared to maternal and control serum (P < 0.001). In vitro degradation of cord IR-TRH LM could not be demonstrated, neither by cord serum itself nor by adult serum, this in contrast to hypothalamus extracted TRH which was readily degraded by adult serum. Neither the presence of an inhibiting substance, nor methodological factors seem to account for the apparently different behavior of cord IR-TRH. Apparently circulating IR-TRH could be closely related, but still different from tissue extracted and synthetic TRH.