Morphological and functional effects of triiodothyronine on regenerating peripheral nerve

The functional and morphologic consequences of treatment of peripheral nerve lesions with exogenous 3,3',5-triiodo-L-thyronine (T3) are unclear. In this study adult Wistar rats with sciatic nerve crush lesions were treated with 20 micrograms/kg/day of T3. The treated animals gained weight though more slowly than vehicle-treated rats. Ten control and 10 experimental animals assessed 4 days postoperatively for sensory axon regeneration by the "pinch test" showed no significant difference in distance regenerated. Motor function recovery was monitored for 21 days after nerve crush lesions by the vestibular placing response and also showed no significant difference. Light microscopy of transverse epoxy sections of the sciatic nerve 5 mm distal to the lesion showed no significant difference in whole nerve area or myelinated fiber density but quantitative electron microscopy revealed a significant increase in axolemma, myelin spiral length, whole nerve fiber area, and nerve fiber perimeter in T3-treated rats. Statistically nonsignificant increases in axonal area and myelin periodicity were also noted. These results are interpreted as indicating that although T3 treatment is not functionally beneficial in the regeneration of adult mammalian peripheral nerve it nonetheless has subtle morphologic effects that indicate a stimulating effect on membrane synthesis.