The FGF21 - induced thirst response appears to depend on another signaling pathway in the hypothalmus, the? (sciencedaily.com)
When the hypothalmus decides we need more thyroid hormone in circulation (cold weather or increased activity level for example) it sends a chemical messenger called thyrotropin - releasing hormone (TRH) which goes to the pituitary gland. (drjockers.com)
The thyroid responds to messages from the hypothalmus and pituitary gland in order to get out and interact with the cells of the body. (drjockers.com)