Synthetic TRH was injected intravenously or into the lateral ventricle of rats and its effect on TSH secretion was estimated indirectly by observing changes in thyroid radioiodine release. Dose-related increases in 2- hr blood 131I levels were observed after TRH injection by either route and injections into the lateral ventricle were no more effective than intravenous injections. TRH was inactivated by preincubation of plasma, but not by preincubation with cerebrospinal fluid (CSF). The recently suggested hypothesis of a route of transport of releasing factors from cerebrospinal fluid to pituitary is not supported by data regarding TRH effects obtained in this study.
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