Hypothalamic portal blood immunoreactive TRH in the rat: lack of effect of hypothyroidism and thyroid hormone treatment.

Possible thyroid hormone regulation of hypothalamic thyrotropin-releasing hormone (TRH) secretion was studied by measurement of hypophyseal portal blood TRH concentrations in normal, hypothyroid and thyroid hormone treated rats. TRH was measured by radioimmunoassay of methanol extracts of blood. Extraction recovery was good. Endogenous TRH immunoreactivity (IR-TRH) was destroyed by incubation with normal human serum and it co-migrated with synthetic TRH in a thin layer electrophoresis and chromatography system. Mean portal blood IR-TRH concentrations ranged from 1041 to 2834 pg/ml in the various groups of rats studied. No consistent differences were found in portal blood IR-TRH concentrations in normal, hypothyroid or thyroid hormone treated rats. Peripheral blood IR-TRH concentrations were similar in normal rats and rats subjected to forebrain and hypothalamic removal. These results indicate that TRH secretion into the hypothalamic-pituitary portal system is not regulated by thyroid hormone and that forebrain and hypothalamic secretion do not contribute importantly to peripheral blood TRH.