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 uptake of thyroid hormone into target tissues. With the discovery of 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 T3 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 and OATPs, and the effects of genetic variation in these transporters.