Thyroid hormones require transporter proteins that facilitate their influx and
efflux through the cellular plasma membranes. There are many families of thyroid
hormone transporter proteins, most of which transport other substrates, including bile
acids, amino acids, monocarboxylates, and organic anions. The only transporter
specific for thyroid hormones is the monocarboxylate 8 transporter or MCT8. MCT8 is
present in the brain barriers and the membranes of neural cells. MCT8 mutations cause
the Allan-Herndon-Dudley syndrome, described in the next chapter. Besides MCT8,
the amino acid transporters LAT1 and LAT2 might have a physiological role in T4 and
T3 transport. The organic anion transporter polypeptide 1C1 or OATP1C1 is a T4
transporter present in the mouse, but not the human, blood-brain barrier, and facilitates
T4 transport to astrocytes and radial glia expressing type 2 deiodinase. A
neurodegenerative disorder in a patient has been attributed to an OATP1C1 mutation.
This chapter describes the physiological aspects of thyroid hormone transport across
the different transporter families.
Keywords: Brain barriers, BBB, Cerebrospinal fluid, CSF, Choroid plexus, LAT1, LAT2, MCT8, OATP1C1.
