I've tried this 2x in the past. The headaches go away after 3 days & then all is well. As soon as I start again, the headaches come back.
This has happened with t3 as well in the past, even at 12.5mcg a day.
This happens as your body adjusts. Sometimes the fillers and biding agents used in the pill cause the side effects you speak of.
But if things don't improve over a period of 2wks or so, then it's best that you stop all together.
100mcg/day t4 is a big dose to start, try 50mcg/day and after two weeks up the dose if you wish.
Same with t3, 12.5mcg/day for a week, then 25mcg, then more if you wish.
Thyroid hormone is very tricky, and people experiences vary due to individual tolerance/response.