Synthroid should be used only by the patient for whom it is prescribed. Synthroid is the number one prescribed treatment for thyroid disease, in conditions where the thyroid gland does not produce enough thyroid hormone. Thyroid hormones come in a variety of reasons, when the body may not produce enough of thyroid hormone on its own, Synthroid is taken to replace the body's thyroid hormone. If you are hypersensitive to thyroid hormone. Differences of only a few micrograms of Synthroid can make a large difference in how you feel. It may take a few weeks for Synthroid to begin working, and you may not see any change in your symptoms until then.