This paper seems to confirm what the MHRA stated in their report on levothyroxine after the Teva incident. That is, oxygen affects levothyroxine tablets. Specifically, oxygen and the amount of water bound to the levothyroxine molecules together have an even greater effect.
Not only can these cause degradation, but we have to ask what the levothyroxine will be degraded into? Could that sometimes be into substances with significant bio-activity themselves?
Pharm Dev Technol. 2013 Dec 3. [Epub ahead of print]
Impact of hydration state and molecular oxygen on the chemical stability of levothyroxine sodium.
Hamad ML, Engen W, Morris KR.
Department of Chemistry, College of Arts and Sciences, University of Hawai'i at Hilo , Hilo, HI , USA and.
Abstract Levothyroxine sodium is an important medication used primarily for treating patients with hypothyroidism. Levothyroxine sodium tablets have been recalled many times since their 1955 introduction to the US market. These recalls resulted from the failure of lots to meet their content uniformity and potency specifications. The purpose of this study is to test the hypothesis that the chemical stability of levothyroxine sodium pentahydrate is compromised upon exposing the dehydrated substance to molecular oxygen. The impact of temperature, oxygen and humidity storage conditions on the stability of solid-state levothyroxine sodium was examined. After exposure to these storage conditions for selected periods of time, high performance liquid chromatography (HPLC) was used to quantify the formation of impurities. The results showed that levothyroxine sodium samples degraded significantly over a 32-day test period when subjected to dry conditions in the presence of molecular oxygen. However, dehydrated samples remained stable when oxygen was removed from the storage chamber. Furthermore, hydrated samples were stable in the presence of oxygen and in the absence of oxygen. These results reveal conditions that will degrade levothyroxine sodium pentahydrate and elucidate measures that can be taken to stabilize the drug substance.
PMID: 24295156 [PubMed - as supplied by publisher]
As so very often, the rest of the paper is behind a paywall.
I also happened to find a PowerPoint all about levothyroxine, stability, etc. - though be warned, it is entirely based on the USA. Note particularly that it mentions "carbohydrate excipients" which would include lactose, maize starch and sucrose - all of which are included in one or more UK makes of levothyroxine.
Image is: Contents of an oxygen absorber contained within a packet of Beef Jerky - often based on iron powder, sodium chloride (common salt) and possibly activated carbon.