Since you were put on Levothyroxine previously I assume you were diagnosed as hypothyroid. So now you have been diagnosed as hyperthyroid??? I expect it is because you were tested for TSH and it came back as suppressed. Is that the case?
Thyroid hormones control the metabolic rate (the amount of energy expended while at rest) so when there is too much thyroid hormone this will cause the metabolic rate to speed up - hyperthyroidism. When there is not enough thyroid hormone the metabolic rate slows down - hypothyroidism.
The treatment for hypothyroidism is to take thyroid hormones to increase the metabolic rate. Levothyroxine (also called L-thyroxine or T4) is a synthetic form of the thyroid hormone thyroxine and is used to treat hypothyroidism.