For the normal cardio vascular function thyroid hormone is very important. Therefore, when enough hormone is not present neither the heart nor the blood vessels can function normally.  In Hypothyroidism the heart muscle becomes weak both in its contraction phase and also in the relaxation phase.  With the result the heart cannot pump vigorously as it used to be and the amount of blood ejected in each heart beat is also reduced.  Besides, as the heart muscle does not relax normally in between heart beats, a serious condition called diastolic dysfunction may result.  Further, the amount of nitric oxide in the lining of the blood vessels is reduced and thereby causing them to stiffen. People with Hypothyroidism, the cardiac symptoms can be seen.