PURPOSE: To assess the local effect of triiodothyronine (T3) on peripheral nerve regeneration in a rat model of sciatic nerve transection. MATERIALS AND METHODS: Forty-five male healthy white Wistar rats were divided randomly into 3 experimental groups (n = 15): sham operation, control (CHIT), and T3 treatment (CHIT/T3). In the sham-operated group, the left sciatic nerve was exposed under anesthesia through a gluteal muscle incision and the muscle was sutured after homeostasis. In the CHIT group, the left sciatic nerve was exposed the same way and transected proximal to the tibioperoneal bifurcation, leaving a 10-mm gap. Each proximal and distal stump was inserted into a chitosan conduit, which was filled with phosphate buffered solution 10 μL. In the CHIT/T3 group, the defect was bridged using a chitosan conduit filled with T3 10 μL. Each group was subdivided into 3 subgroups of 5 animals each and studied 4, 8, and 12 weeks after surgery. Data were analyzed statistically by factorial analysis of variance and the Bonferroni test for pairwise comparisons. RESULTS: Behavioral testing and sciatic nerve function study confirmed a faster and better recovery of regenerated axons in the CHIT/T3 group than in the CHIT group (P < .05). Gastrocnemius muscle mass was significantly larger in the CHIT/T3 group than in the CHIT group. Morphometric indices of regenerated fibers showed that the number and diameter of the myelinated fibers were significantly larger in the CHIT/T3 group than in the CHIT group. Immunohistochemistry showed that the locations of reaction to S-100 were clearly more positive in the CHIT/T3 group than in the CHIT group. CONCLUSIONS: The response to local treatment showed that thyroid hormone influenced and improved the functional recovery of peripheral nerve regeneration.