Thyroxine half life & dose changes

[Curious123]

I am aware of the waiting 6-8 weeks after a dose change etc before testing levels.

I was wondering if anyone knows of a graph or paper that discusses the expected hormone concentration level from expected synthetic FT4 decline versus the increased thyroid hormone production after a dose decrease?

I believe the half life is around a week so it takes approximately 4 weeks give or take based on individual metabolism for thyroxine to exit the body. So effectively 1 week after stopping thyroxine for example, your synthetic FT4 concentration should be half what it was, but in theory a well thyroid would start to kick in to produce more thyroxine. Does anyone know how long the feedback loop takes in theory to start the increased natural thyroid hormone production?

[tattybogle]

TSH is very fickle..... how long it takes to rise (and therefore stimulate the thyroid to try and make more thyroid hormone again) depends on many variables .

how low it was , how long it's been low , why it was low /or high in the first place etc etc .... so theory is probably not much use here . in practice you'll see all sorts ... often pretty inexplicable eg in some it can shoot up pretty quick within a couple of weeks and in others it can take several months to rise much at all.

[HealthStarDust]

We are often told it is 7 weeks ish. According to Wikipedia it could be almost 10 weeks.

[Jazzw]

I can only speak from my own experience.

As some of us do, I once decided to see what would happen if I stopped taking thyroid hormones.

Three weeks in, my TSH was still completely suppressed and my FT4 and FT3 were both several points below the bottom of their ranges.

I’m not sure how long I’d have had to wait for my TSH to wake back up again and prompt my hibernating thyroid gland to have a go at making some of its own juice because I panicked and went back on my NDT.

I think the answer is, it depends. For some, you’d see a response in days. For others, it takes quite a while.