Thyroid hormone is a pleiotropic hormone with widespread biological actions. For 
instance, adequate levels of thyroid hormone are critical for the development of 
different tissues such as the central nervous system, but are also essential for 
the regulation of metabolic processes throughout life. The biological activity 
of thyroid hormone depends not only on serum thyroid hormone levels, but is also 
regulated at the tissue level by the expression and activity of deiodinases, 
which activate thyroid hormone or mediate its degradation. In addition, thyroid 
hormone transporters are necessary for the uptake of thyroid hormone into target 
tissues. With the discovery of monocarboxylate transporter 8 (MCT8) as a 
specific thyroid hormone transporter and the finding that mutations in this 
transporter lead to a syndrome of severe psychomotor retardation and elevated 
serum 3,3',5-tri-iodothyronine levels known as the Allan-Herndon-Dudley 
syndrome, the interest in this area of research has greatly increased. In this 
review, we will focus on the molecular aspects of thyroid hormone transporters, 
including MCT8, MCT10, organic anion transporting polypeptides, and the effects 
of genetic variation in these transporters.