The role of transthyretin in the transport of thyroid hormone to cerebrospinal fluid and brain.

Cerebrospinal fluid (CSF) transthyretin (TTR), the main CSF thyroxine (T4) carrier protein in the rat and the human is synthesized in the choroid plexus (CP). After injection of 125I-T4 in the rat, radioactive T4 accumulates first in the CP, then in the CSF and later in the brain. We have shown that competitive inhibition of T4-TTR binding sites in the CP and CSF by a flavonoid results in a decrease in the radioactivity present in the brain after i.v. injection of 125I-T4, suggesting that T4 transported to the CSF via the CP is a significant pathway for the supply of thyroid hormones to the brain.