What Causes Spike In Energy Level After Reducing Thyroxine Dosage?

Hi,my doctor reduced my thyroxin from 100mg.to 75mg.it is a under active thyroid I have,I don t feel tired any more and have loads of energy,I have been on the reduced dose for 2 weeks now.i don t go back for bloods until 10 weeks.is this normal, and was I tired and lethargic as I was taking 100mg.