Researchers now have a better understanding of the role that thyroid hormones, the tissues that produce them, and the biochemical pathways on which they act have in driving seasonal reproduction in some mammals, and how this new information may help explain seasonal changes in metabolism and mood that affect humans. The review article entitled “Seasonal Rhythms: The Role of Thyrotropin and Thyroid Hormones” is part of a special section on Japanese Research led by Guest Editor Yoshiharu Murata, Nagoya University, Japan, in the January 2018 issue of Thyroid.