Cassia fistula Linn. (Purging cassia, Golden shower, Amaltas) is widely grown as an ornamental plant and well known traditional plant in India. The plant is having antioxidant activity, anti-inflammatory, laxative, wound healing, hepato-protective, antitussive, cardiovascular activity, antiulcer, antibacterial agent, larvicidal, ovicidal and antifertility effect. The aim was to search for anti-hypothyroidism activity of aqueous and ethanolic extract of Cassia fistula leaves. Aqueous and ethanolic extract of Cassia fistula leaves were investigated for its potential to protect hypothyroidism against hypothyroidism induced by propylthiouracil (PTU) in rats. fT3, fT4, TSH, cholesterol level and changes in body weight were used for evaluation of antihypothyroidism activity. Treatment with both extracts of Cassia fistula in dose of 300 mg/kg significantly increased fT3, fT4 and decreased TSH and cholesterol level compared with control group (p>0.001).Based on the studies we concluded that the ethanolic and aqueous extracts of Cassia fistula leaves might contain some active principles against hypothyroidism.