How are you taking the cynomel? Nibbled throughout the day? All at once? With meals?
I find that after a while taking cynomel by itself didn't raise my temperature that much, and taking more definitely made things worse - headaches, fatigue, etc.
I think if you are using a lot of T3 but don't have adequate cofactors - Vitamin A, Cholesterol, copper, fructose etc., then it can be very stressful to you.
From DannyRoddy's Ray Peat's Brain Article"
"With any product, a single dose of T3 of about 4 mcg is close to the physiological range; sometimes a smaller amount is enough."