Thyroid hormone (TH) is essential for normal development and growth of the central nervous system (CNS), and neuronal regeneration in adults after traumatic brain injury (TBI). The entry of thyroxine (T4) into brain is via two main barriers, the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) barrier through the choroid plexus (CP). We have previously demonstrated that transport of T4 from CSF into CP and other brain tissues is carrier-mediated and dependent on the presence of transthyretin and on organic anion transporting polypeptides. However, the transport route of T4 from the CSF compartment into blood across CP is still poorly investigated. The aim of this research was to characterize the mechanism by which T4 is transported from CSF to blood across the lateral ventricle CPs in situ. We investigated the effect of taurocholate and of sodium replacement with choline in artificial CSF (aCSF) on 125I-labelled-T4 transport at the apical (CSF) side of the CP using the steady-state method to calculate 125I-T4 efflux from the CSF to blood (R%). Sheep (Clun Forest strain, either gender) weighing 20-45 kg were anaesthetised with sodium thiopental (20 mg/kg, iv), heparinized (10,000 U, iv), exsanguinated, and head was removed. The brain was extricated from the skull and the CPs perfused with a modified Ringer solution via internal carotid arteries at a flow rate of 1.5 ml/min. Both ventricles were then opened, and the CP was superfused with aCSF containing 0.018MBq 125I-labelled T4 and 0.37MBq 14C-labelled mannitol (as a reference molecule) per 100 ml of aCSF. After 1 h, steady state uptake of 125I-labelled T4 was achieved, then the CP was perfused with low Na+ aCSF (contained 10 mM Na) or aCSF containing low Na+ and 1.0 mM taurocholic acid (TA) for 40 min, followed with normal aCSF. Values are means ± S.E.M., compared by t Test. Results showed that the R% in the presence of low Na+-aCSF (1.8±0.1%) was significantly attenuated (n=3, p<0.05) compared to control (3.1±0.6%) (n=3). On the other hand, this efflux decreased even further after the addition of TA (0.54±0.11%), an inhibition of > 70% in comparison to low Na+ (n=3, p<0.01). We suggest that the reduction in the R% of T4 efflux from the CSF to blood across the CP epithelial cells in the presence of low Na+ and TA might be mediated by Na+-Taurocholate cotransporting polypeptide (Ntcp) functioning at the apical side at of the CP by controlling T4 concentration in the CSF compartment and monitoring the efflux toward the blood. This is consistent with our previous finding which demonstrated the presence of Organic anion transporting polypeptides dependent on sodium gradient at the CSF face of the CP in vivo. This work is in progress to determine the mechanisms of T4 efflux from CSF to blood across blood-CSF-barrier. All procedures accorded with current UK legislation and approved by Lebanese University, NRC, internal board.