The transport of thyroxine from the bloodstream to the brain and the synthesis 
and secretion of transthyretin (formerly called prealbumin) were studied in rats 
and in sheep choroid plexus perfused in vitro. Rat choroid plexus contained 4.4 
micrograms and rat liver 0.39 micrograms transthyretin mRNA per gram wet tissue. 
The specific radioactivity of transthyretin isolated from cerebrospinal fluid of 
rats 60 min after intravenous injection of [14C]leucine was greater than 50 
times that of transthyretin from serum. After adding [14C]leucine to the 
perfusion medium of an in vitro perfused sheep choroid plexus, highly 
radioactive transthyretin was isolated from freshly secreted cerebrospinal fluid 
collected from the exposed choroid plexus surface. Secretion of newly 
synthesized transthyretin into the perfusion medium could not be demonstrated. 
After intravenous injection of [125I]-thyroxine into rats, a maximum in the 
curve of radioactivity in tissue plotted against time after injection was 
observed first for choroid plexus, thereafter for cerebrospinal fluid, and still 
later for cortex and striatum. Based on the obtained data, a hypothesis is 
derived for the mechanism of the transport of thyroid hormones from the 
bloodstream to the brain involving transthyretin synthesized in choroid plexus 
and secreted into the cerebrospinal fluid.