SUMMARY 
The foliowing cardiac effects may be at­tributed to thyroxin: 
1-Thyroxin augments all anaerobic pro­cesses in the body includ::ng the heart, and decreases the glycogen content of the heart ( 1, 2, 5, 27). The resistance of the heart to anoxia is increased in hyperthyroidism ( 25).

2- Thyroxin influences the cardiac weight and prevents cardiac atrophy (3, 8, 10, 11, 19, 20, 21, 30, 33). 
3-Thyroxin stimulates synthesis of a new cardiac protein and possibly a newcardiac myo­sin (I 0, 16, 25, 35, 36, 38). 
4-Thyroxin influences the inherent con­tractility of the heart muscle, and this influence is inden􀂡ndent of the norepinephrine stores and the alterations in the high energy phosphate stores ( 6). Thyroxin has an inotropic and a chro­notropic action on heart muscle (14). 
5-Manifestations of the thyroxin effects on the heart are the same as the manifestations of the stimulation of beta-adrenergic receptors. These effects could be blocked by beta-adrenergic blocking agents ( 32). These beta-adrenergic re­ceptors are probably adenyl cyclase (31). 
6-Increased levels of epinephrine and norepinephrine, and stimulation of beta receptors may be involved in the manifestations of thy­roxin effects in hyperthyroidism. ( 14)