ABSTRACTThe effects of orally and topically administered thyroxine on hair growth were studied in normal, intact, cross‐bred male dogs maintained under the same controlled conditions of temperature and photoperiod. It was found that, irrespective of the route of administration, thyroxine produced an increase in both the rate of hair growth and in the number of hair follicles entering the anagen phase of the hair cycle. With orally administered thyroxine, the changes in hair growth were most marked in the flank regio