The major goals of this study were to determine the effects of different doses of thyroxin on the rate of orthodontic tooth movement and the force-induced root resorption. In this study fifty male Sprague--Dawley rats were divided into five groups: a normal group with no intervention; a control group in which appliances were placed and 10 ml/kg i.p./day normal saline was injected; and three thyroxin groups in which appliances were placed and 5, 10 and 20/microgram/kg i.p./day L-thyroxin were administered, respectively. A fixed orthodontic appliance consisting of a 5 mm closed-coil spring was ligated between the maxillary incisor and maxillary first molar to deliver an initial force of 60 gm. The results showed that administration of 20/microgram/kg i.p./day L-thyroxin significantly increased the amount of orthodontic tooth movement (p < 0.001). The extent of root resorption as seen from scanning electron micrographs decreased with thyroxin administration.