After taking a break from cycling during the winter due to colds, flu, and feeling very run-down I started riding again in February but found it very hard going. Despite getting the miles in my average speed was the worst its been compared to identical rides I did in previous years. I had no go in my legs (cadence 50 instead of usual 65+), and poor recovery in the following days following cycling.
After getting a letter about Teva Levothyroxine from my GP I checked my pills and found that's what I'd been taking, last time I remember looking at the brand (last autumn probably) I was on it had been Actavis. After binning the Teva pills (as advised by my pharmacist) and changing back to Actavis Levothyroxine (200mcg per day) a couple of weeks ago my cycling performance has jumped back to normal, going by the cycling log I keep.
I will get a blood test done in a few weeks and ask my GP to specify the same Actavis brand on my repeat prescription if possible. I'm annoyed I didn't notice the brand had changed and I failed to connect feeling run down over winter to my thyroid treatment. Years ago I noticed I felt worse after changing from the Eltroxin brand to different ones (APS and Cox I think), after testing my dose was adjusted.