The effects of thyroid hormones on the skin of beagle dogs

The effects of hypothyroidism on canine skin were determined by comparing m orphologic, morphometric, and hair cycle differences in skin biopsy samples from 3 groups of age- and gender-matched Beagle dogs: (1) euthyroid dogs; (2) dogs made hypothyroid by administration of I-131; and (3) dogs made hyp othyroid and maintained in a euthyroid state by treatment with synthetic th yroxine. After 10 months of observation, there was slower regrowth of hair 2 months after clipping in the untreated-hypothyroid dogs. Untreated-hypoth yroid dogs had a greater number of follicles in telogen and fewer hair shaf ts (ie, a greater number of hairless telogen follicles) than did the contro l group. The control dogs had a greater number of telogen follicles but the same number of hair shafts as the treated-hypothyroid group. Treated-hypot hyroid dogs had the greatest number of follicles in the growing stage of th e hair cycle (anagen). This study suggests that, at least in Beagles, induc ed hypothyroidism does not affect the pelage as dramatically as has been de scribed in naturally occurring disease. This is because normal Beagles reta in hair shafts in follicles for long periods, and the alopecia of hypothyro idism appears to evolve slowly because of the prolongation of this haired t elogen stage. The evaluation of thyroxine-treated hypothyroid dogs demonstr ates that thyroid hormone supplementation of Beagle dogs with induced hypot hyroidism stimulates hair growth.