Confused by 'levothyroxine replaces; it doesn't top up'

[BornFlippy]

Hello all,

I have Hashimoto's, and take 100mg of Levothyroxine.

I've just ordered a bloodtest from BlueHorizon, as I'm wracked with fatigue again.

Before I get the results, I was hoping for some clarificiation on the fact I've seen stated in some posts about Levothyroxine being a replacement, not a top up. I've been on the same Levothyroxine does for over 10 years - I'm assuming that as I age, my thyroid is atrophying (due to Hashimoto's), and so it's likely that I will need an increased dose over time? Sorry - I know that's a very basic question.

I find talking to doctors very intimidating, so I'm trying to get a better grasp of the basics.

Thanks in advance : )

[helvella Administrator Thyroid UK]

Imagine your thyroid is producing not quite enough thyroid hormone. Maybe you see a small rise in TSH, and FT4 is a touch lower than seems ideal, and FT3 similarly.

Imagine you do a whole load of calculations based on the basic chemistry, the biochemistry, the biology, etc., and see that your thyroid is producing the equivalent of 25 micrograms less T4 than you need.

Now you start taking 25 micrograms of levothyroxine every day. Which might look as if it would exactly make up that shortfall from your own thyroid.

But once you start taking that dose every day, your TSH drops, and so your thyroid will make LESS thyroid hormone.

You were just 25 micrograms down on what you need because your TSH rose and demanded your thyroid made more thyroid hormone. Now your thyroid's output has dropped further. So you need to take more than 25 micrograms.

And this just keeps turning the screw - reducing what you make and increasing what you need to take.

This is the issue that is being highlighted by the "replaces ... not tops up" argument.

However, if that were it, it would be simpler. I, for example, take 125 micrograms a day. If you tried to estimate how much I would need as a full replacement dose (say I had a total thyroidectomy), I think almost everyone would think I need more than 125. It appears that while my thyroid's output is well down on what it should be producing, it must still be making some thyroid hormone. So the "replaces" argument starts out being quite reasonable, but it doesn't necessarily hold to the extreme of seeing your thyroid stopping all output.

[BornFlippy in reply to helvella]

thanks so much Helluva - much appreciated : )

[jimh111]

It's a bit of both! I suspect the balance depends on how the thyroid fails. Imagine, if we cut off a quarter of your thyroid and gave you a quarter of your normal T3 and T4 secretion (with magical 100% absorption) your hormones would be replaced. Thyroid tablets are topping up.

However, the thyroid may degrade, it may need more whacking from TSH to secrete, like a car firing on five instead of six cylinders. Now if we give you the same amount of hormone your TSH would fall as expected but may not be enough to whack your degraded thyroid to secrete the three quarters it was before you were given the hormone tablets. So you need a little more. It tops up and replaces!

I don't agree with the statement it replaces, at least not when there is still some residual thyroid function. Certainly when the thyroid is caput it is fully replacing the hormone.

Hope this isn't too complicated. I don't think it is a concept that is worth worrying about.

[BornFlippy in reply to jimh111]

thanks Jimh11 - good analogy there