When reducing Synthroid dose, how fast does blood level fall?

[5states]

I just reduced my Synthroid dose and I am wondering how long it will take before my blood level reflects the new dose.  I know that when increasing a dose of Synthroid, it takes six weeks before the blood level fully reflects the new dose. Does it work the same way when reducing the dose, i.e. it takes 6 weeks to fall to the new level?  Also, since the half-life of Symthroid is 1 week, can I expect that after one week on the new dose my blood level will be halfway between the old level and the (future) new level?

[jennerfennerbenner]

i can tell a difference within 2 weeks

[gimel]

As I understand the effect of half life of T4, the answer is yes.  You can expect that after a week your blood levels will reflect about 50% of the change, then 25% the following week and 12.5% the next and 6.25% the next week.  So after 4 weeks the net effect should be about 90% of the change in medication.