Transthyretin (TTR) is a protein that binds and distributes thyroid hormones 
(THs). TTR synthesised in the liver is secreted into the bloodstream and 
distributes THs around the body, whereas TTR synthesised in the choroid plexus 
is involved in movement of thyroxine from the blood into the cerebrospinal fluid 
and the distribution of THs in the brain. This is important because an adequate 
amount of TH is required for normal development of the brain. Nevertheless, 
there has been heated debate on the role of TTR synthesised by the choroid 
plexus during the past 20 years. We present both sides of the debate and how 
they can be reconciled by the discovery of TH transporters. New roles for TTR 
have been suggested, including the promotion of neuroregeneration, protection 
against neurodegeneration, and involvement in schizophrenia, behaviour, memory 
and learning. Recently, TTR synthesis was revealed in neurones and peripheral 
Schwann cells. Thus, the synthesis of TTR in the central nervous system (CNS) is 
more extensive than previously considered and bolsters the hypothesis that TTR 
may play wide roles in neurobiological function. Given the high conservation of 
TTR structure, function and tissue specificity and timing of gene expression, 
this implies that TTR has a fundamental role, during development and in the 
adult, across vertebrates. An alarming number of 'unnatural' chemicals can bind 
to TTR, thus potentially interfering with its functions in the brain. One role 
of TTR is delivery of THs throughout the CNS. Reduced TH availability during 
brain development results in a reduced IQ. The combination of the newly 
discovered sites of TTR synthesis in the CNS, the increasing number of 
neurological diseases being associated with TTR, the newly discovered functions 
of TTR and the awareness of the chemicals that can interfere with TTR biology 
render this a timely review on TTR in neurobiology.