I have been diagnosed with an underactive thyroid a year ago. I'm on 75mg of Levothyroxine and I'm largely feeling much better. I feel great most of the time, but every 3 weeks or so (no set pattern, just general observation) I get a few bad/tired days. The kind where I'm exhausted when I wake and it's lasts all day (brain fog/muscle aches) I'd just like to know if this is normal and is something I just have to get used to having an underactive thyroid?! Like I say, Most of the time I feel great and I've just taken up the c25k challenge which has made me feel amazing. I just hate those bad days 