TTR and bipolar disorder: TTR is a homotetrameric protein synthesized by the liver, retinal pigment epithelium and choroid plexus.41 Present in cerebrospinal fluid (CSF), TTR transports tetraiodothyronine (T4) across blood–brain barrier.42TTR locus, 18q12, has been implicated in bipolar disorder in a Danish pedigree of bipolar patients.43 Chronic administration of lithium to rats caused a 16-fold decrease in TTR,44 which differs from our data that suggest lithium-treated patients had an increase in TTR.