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The Changes in your blood pressure redings are significant and is called postural hypotension
or orthostatic hypotension
,it should be investigated for cause and lightheadedness
could be because of this.
To know any such defect in the heart one should get Echocardiography
along with color doppler study of heart, after knowing exactly what kind of defect is there in heart treatment can be planned.
Continue to take your thyroid medicine and let us wait for the test result before any thing can be decided.
Good Luck to You