Figure 3.

Diagramatic representation of the hypothalamic–pituitary–thyroid axis with positive regulation (black) predominantly mediated by thyrotropin-releasing hormone (TRH) and negative (grey) feedback influences, predominantly mediated by thyroid hormone receptor (TR) isoforms β2 and β1. Putative transporter molecules (grey) mediating these effects are annotated. OATP1C1 is expressed in capillaries throughout the brain, monocarboxyate transporter 8 (MCT8) is expressed in the PVN of the hypothalamus and in follicular stellate cells in the anterior pituitary (reviewed in Fliers et al. (2006)). Tissue-specific TR isoform expression is described, for thyroid hormone target tissues.