Intrathecal triiodothyronine administration causes greater heart rate stimulation in hypothyroid rats than intravenously delivered hormone. Evidence for a central nervous system site of thyroid hormone action

To determine whether intracerebrally localized iodothyronines produce thyroid hormone-related functional effects, heart rate responses were compared in conscious hypothyroid rats given triiodothyronine (T₃) by either the intrathecal or the intravenous route. A significant increase in heart rate occurred within 18 hours after 1.5 nmol T₃/100 g body wt was delivered intrathecally through a cannula previously placed in the lateral cerebral ventricle. Injection of the same T₃ dose intravenously through an indwelling jugular catheter or injection of vehicle only by either route produced no significant increase in heart rate during the 48-hour postinjection period of observation. These differences were observed even though integrated serum T₃ concentrations were significantly lower after intrathecal than after intravenous T₃ injection. The results indicate that thyroid hormone effects on heart rate are exerted within the brain as well as within the heart.