Probably just a rise in metabolism. Same thing can happen when taking thyroid, however you eventually adjust to the effects of thyroid. Not sure about "high-dose" thiamine. Thiamine as opposed to thyroid will carry the additional risk of lower potassium levels.
In regards to the adrenaline issue just eat more carbs and salt. If that doesn't cut it you could try also adding taurine, niacinamide, inosine, or progesterone. Basically anything that will blunt adrenaline until your metabolic "set-points" adjust to the new normal.