POMC and Cushing syndrome: The administration of triiodothyronine (T3) and T4 suppressed the basal and ACTH-stimulated levels of blood cortisol, at least in rats [23]; in contrast, low levels of thyroid hormones could lead to a chronic elevation of endogenous blood cortisol and thus impaired immune function [22], although a previous study revealed that the immune response of dogs with treated hyperadrenocorticism (HAC) to MLV vaccination against CPV was not significantly impaired in comparison to that of healthy dogs [25].