TH and hypothyroidism: In contrast, hypothyroidism induced by long-term administration of propylthiouracil (PTU), which inhibits the production of new TH, in juvenile female mice results in delayed pubertal onset, with increased hypothalamic GnIH expression and decreased pituitary–gonadal activity; the effect of hypothyroidism in delaying pubertal onset is prevented by GnIH KO [52].