A study of the canine pituitary-adrenal-thyroid axis

Abstract

The pituitary-adrenal-thyroid axis was studied in a series of 32 (l6 male and 16 female) healthy, sexually intact beagles. The dogs exhibited a difference between sexes with regard to total serum thyroxine (T4) and cortisol levels. However, since neither adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), nor the plasma concentrations of thyroxine- and cortisol-binding proteins were determined for the dogs that were used, the possible reasons for a sex difference are inconclusive. Because of this, studies need to be made on the effects of the sex hormones on the total binding capacities of the thyroxineand cortisol-binding proteins as well as their effects on ACTH and TSH. The total serum levels of triiodothyronine (T3) never changed significantly despite significant changes in thyroxine and cortisol concentrations. Since the explanation for this is somewhat speculative, more research needs to be conducted on the maintainence of serum T3 levels in the presence of lowered concentrations.