Accurate data concerning the effect of hyperthyroidism on the size of the heart do not appear to be available in the literature. Not only is the heart rate markedly increased in this condition but, according to Samson Wright,1 the stroke volume in exophthalmic goiter averaged 116 cc., as compared with the normal of from 70 to 120 cc. at rest; the minute output averaged 11.7 liters as compared with from 5 to 8 liters in normal persons at rest. The increased output of the heart may not be due to the tachycardia per se, for Stewart and Crawford2 induced regular tachycardias by means of single induced break shocks thrown into the auricle at a regular rapid rate of between 250 and 400 per minute, and found that the blood flow was usually unchanged and in one third of the observations it was decreased.
Observations on the size of the heart
SIMONDS JP, BRANDES WW. THE SIZE OF THE HEART IN EXPERIMENTAL HYPERTHYROIDISM. Arch Intern Med (Chic). 1930;45(4):503-512. doi:10.1001/archinte.1930.00140100025002