FDA tightens potency restrictions for levothyroxine

October 5, 2007

FDA makes changes to levothyroxine potency specifications

Concerns that the potency of levothyroxine may deteriorate faster than its expiration date have prompted the FDA to change the requirement for the drug to meet a 95% to 105% potency specification. The recommendation for the change came from a joint FDA advisory committee after complaints about the drug's potency were received by patients and healthcare professionals. The agency requested stability data from each manufacturer of all approved levothyroxine preparations. The data showed a trend toward loss of potency with some products approaching 90% of labeled potency by the expiration date. Current specifications say that the drug must fall within a 90% and 110% range, and all tested brands did; however, some products degraded more rapidly over their shelf life than others. The new range will ensure that no product degrades by more than 5% before expiration. The upper specification of 105% is appropriate to address occasional analytical testing variability. More information can be found at