Experimental hypothyroidism increases immobility in rats in the forced swim paradigm

Effects of severe and mild hypothyroidism on the immobile response to inescapable stress were examined in male Wistar rats using the forced swim paradigm. Rats were exposed to two sessions of inescapable swim stress: pretest (for 15 min) followed by test (for 5 min) 24 h later. Surgically thyroidectomized rats showed a significant increase (by 90%) in immobility during test compared to sham rats. Chronic administration of high (200 micrograms/kg per day) but not low (15 micrograms/kg per day) dose of T4 prevented the increase in immobility in thyroidectomized rats. Normal rats submitted to iodine-free diet for 2 weeks in order to produce a mild hypothyroidism showed a significant increase (by 60%) in immobility time during test compared to control rats. The results indicate that hypothyroid rats are more vulnerable to inescapable stress than normothyroid rats.