The Food and Drug Administration has directed manufacturers of levothyroxine (Synthroid and others), a drug used by more than 13 million patients to treat thyroid disorders, to adjust manufacturing procedures to ensure that the product meets a standard potency specification of 95% to 105% through the expiration date. Formerly, the standard minimum potency was 90%, but certain formulations have been found to deteriorate more rapidly than others, losing therapeutic effectiveness while still available for sale. With the higher minimum potency standard, the potency should remain at therapeutic levels even after the loss of 5%. The manufacturers have been given two years to comply with the directive. Nurses should instruct patients who take levothyroxine to store it at room temperature in a dry environment, because heat and humidity promote its deterioration.

FDA acts to ensure thyroid drugs don't lose potency before expiration date. FDA News 2007 Oct 3. http://www.fda.gov/bbs/topics/NEWS/2007/NEW01717.html.