My doctor reduced my euthyrox 100 to 50 since that time i feel tired and i have fast heart beats, he said i have under active thyroid with nodules?

Underactive thyroid: It should be easy to check whether you're getting enough thyroid hormone. Have your doctor (if s/he hasn't already) order a TSH test. If the TSH test is high, you need more thyroid hormone. The nodules usually do not have any effect on thyroid hormone levels (with the single exception of a toxic nodule, which makes your thyroid overactive).