Periodontal 5'-deiodination on forced-induced root resorption--the protective effect of thyroid hormone administration.

The present investigation was designed to study the protective effect given by thyroid hormone (TH) on root resorption: (1) whether intra-peritoneal versus oral TH administration had the same efficiency; and (2) whether this effect involved local or systemic mechanisms. For this purpose, circulating T3 levels, systemic alkaline phosphatase (APase) activity, and 5'deiodinase (5'D) activity were evaluated in the periodontal area of 80 Sprague-Dawley rats, 8 weeks of age, in which orthodontic appliances had been inserted. The results showed that TH-treated animals (intra-peritoneal or oral) had significantly less force-induced root resorptive lesions compared with a control group, without apparent changes in T3 or APase levels, and that periodontal remodelling was accompanied by a significant increase in local T3 generation as a result of T4 deiodination. This 5'D activity was higher in those animals that received exogenous TH. These results suggest that this protective TH mechanism may be achieved at a local level and that administration of low doses of TH may play a protective role on the root surface either during orthodontic treatment or in those patients that present spontaneous root resorptive lesions.