I've done a number of 20- 24hr + events (400 - 600k brevets, double centuries that take too long, etc.). To be honest, sometimes I just forget to take the thyroid and I figure for a day it won't mess things up too badly. And it doesn't. I might throw in an extra 1/2 pill the next day if I miss a dose. I've been on this so long that I will self-adjust the dose within a slight window to account for where my weight it, etc.

My last 600k, I think I just put it in the saddle bag with the extra shorts I was going to change into after the sleep stop. I take mine at night, so it is a lot more of a cr@pshoot as to whether I will be able to take it or remember when I'm finishing an event at midnight or 2 am.