Directional thyroid hormone distribution via the blood stream to target sites

The thyroid secretes thyroxine (T4) and triiodothyronine (T3), which circulate in the blood stream bound to thyroxine-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). After passing the blood brain barrier (BBB), T4 circulate in the cerebrospinal fluid bound to TTR and T3 mainly bound to apolipoproteinE (ApoE). Degradation of thyroid hormones yields metabolites, the most abundant of which is 3- iodothyronamine (3-T1AM), which binds to apolipoproteinB-100 (ApoB100), which in turn binds to the low density lipoprotein receptor (LDL-R), possibly facilitating cellular uptake of 3-T1AM.220