Explain: Can somehow explain to me how if put on... - Thyroid UK

Thyroid UK

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Explain

Eshep
Eshep

Can somehow explain to me how if put on too low of a dose of synthroid (T4) it can actually make hypo symptoms worse? I would think that maybe a too low dose would leave it unchanged, but how would too little of a T4 dose make you feel worse?

43 Replies
SeasideSusie
SeasideSusieAdministrator

Eshep

It would switch off any thyroid hormone your body makes itself but isn't enough to replace that plus give you the extra that you need to treat your hypothyroidism.

Eshep
Eshep in reply to SeasideSusie

Oh ok, so the little that your thyroid makes if hypo would be signaled to stop producing all together, yet the too low a Synthroid dose wouldn’t be sufficient to alleviate symptoms, so youd still end up feeling poorly and possibly worse?

SlowDragon
SlowDragonAdministrator in reply to Eshep

Exactly

Levothyroxine doesn’t top up failing thyroid, it replaces it. So it’s important to be taking high enough dose

But it’s a concept that’s beyond many medics

guidelines on dose levothyroxine by weight

Even if we frequently don’t start on full replacement dose, most people need to increase levothyroxine dose slowly upwards in 25mcg steps (retesting 6-8 weeks after each increase) until eventually on, or near full replacement dose

NICE guidelines on full replacement dose

nice.org.uk/guidance/ng145/...

1.3.6

Consider starting levothyroxine at a dosage of 1.6 micrograms per kilogram of body weight per day (rounded to the nearest 25 micrograms) for adults under 65 with primary hypothyroidism and no history of cardiovascular disease.

Also here

cks.nice.org.uk/topics/hypo...

gp-update.co.uk/Latest-Upda...

Traditionally we have tended to start patients on a low dose of levothyroxine and titrate it up over a period of months. RCT evidence suggests that for the majority of patients this is not necessary and may waste resources.

For patients aged >60y or with ischaemic heart disease, start levothyroxine at 25–50μg daily and titrate up every 3 to 6 weeks as tolerated.

For ALL other patients start at full replacement dose. For most this will equate to 1.6 μg/kg/day (approximately 100μg for a 60kg woman and 125μg for a 75kg man).

If you are starting treatment for subclinical hypothyroidism, this article advises starting at a dose close to the full treatment dose on the basis that it is difficult to assess symptom response unless a therapeutic dose has been trialled.

BMJ also clear on dose required

bmj.com/content/368/bmj.m41

bestpractice.bmj.com/topics...

Eshep
Eshep in reply to SlowDragon

Thank you! You seem very knowledgeable. Could you please when you have a moment take a look at my most recent post about my thyroid mystery and let me know your thoughts. Doctors are of no help.

Steamruss
Steamruss in reply to SlowDragon

Both of these links appear to need subscriptions?

helvella
helvellaAdministrator in reply to Steamruss

To read the full papers - yes. But the basic information is visible without.

Hi slowdragon

I'm 62 kg and using your dosing formula, I get 99, so I should be on 100 Levo?

Thanks

Alps

SlowDragon
SlowDragonAdministrator in reply to Alpsholiday

Yes....as a guide

Some people need higher doses

Thank you

SeasideSusie
SeasideSusieAdministrator in reply to Eshep

Yes, that's correct.

Let's say that to feel well you need 100mcg thyroxine and you are only making 50mcg, you are hypothyroid with lots of symptoms.

Your doctor gives you 25mcg levothyroxine as a starter dose. This will switch off your own production of thyroxine but isn't even replacing the 50mcg you were previously making, let alone giving you the extra that you need.

Even a starter dose of 50mcg will switch off what you are naturally making but again isn't giving you the extra that you need.

In the UK there seems to be 2 ways of starting patients on Levo.

1) a starter dose of 50mcg (unless a child, elderly or with a heart condition when it should be 25mcg) then retest after 6 weeks and increase by 25mcg, retest and increase every 6-8 weeks thereafter until optimal dose is achieved.

Or

2) start on full replacement dose which is suggested to be weight, ie 1.6 micrograms per kilogram of body weight per day (rounded to the nearest 25 micrograms) for adults under 65 with primary hypothyroidism and no history of cardiovascular disease. Dose adjustment as necessary to reach optimal dose.

Eshep
Eshep in reply to SeasideSusie

Thank you!! When you have a moment, can you please check out my most recent post about my thyroid mystery. I could really use some legit insights.

I don’t understand this as last time I was on 25mcg my TSH went up. Which implies that the pituitary is calling for the thyroid to produce more not switching off production? What am I not understanding?

SeasideSusie
SeasideSusieAdministrator in reply to MiniMum97

If you take exogenous thyroid hormone it shuts off your natural thyroxine production.

If you don't take enough exogenous thyroid hormone for your needs your pituitary will call for more hence TSH rises.

So how is your exogenous thyroid production shut off if not via TSH? What tells the thyroid to stop producing?

SeasideSusie
SeasideSusieAdministrator in reply to MiniMum97

Exogenous thyroid hormone = synthetic thyroid hormone, ie the levothyroxine you take.Taking exogenous thyroid hormone (Levo) shuts off your naturally produced thyroid hormone.

Oh sorry I meant natural thyroid hormone. How is that shut off if it’s not via TSH? What’s the process?

tattybogle
tattybogle in reply to MiniMum97

"I don’t understand this as last time I was on 25mcg my TSH went up"

So in this particular example the extra 25mcg was not enough to lower TSH, (so will not have lowered your own thyroidal hormone production ).

Or to be even more precise ... on the day of that blood test your TSH was higher ... no way of knowing what it did in the first few days /week of taking 25mcg though ... it could have gone down a bit then back up.

The situation we were explaining above would be relevant to someone who's TSH had fallen, as a result of taking levo

MiniMum97
MiniMum97 in reply to tattybogle

Ok so taking thyroid hormone replacement sometimes shuts off your own thyroid production if you have taken enough for it to lower TSH.

OK so what I don’t understand there then is why this happens. In the original question, the poster asked why taking 25mcg only can make you feel worse which is a common reaction.

So your thyroid is producing inadequate amounts of thyroid hormones. Gradually your TSH goes up as your pituitary realises there is not enough thyroid hormone. You eventually are diagnosed as hypo as your GP gives you 25mcg to start you off.

As you start taking it, your body is producing some thyroid hormone but not enough. You take a very small amount of synthetic hormone that is not going to do very much at all to normalise your levels. You still won’t have enough thyroid hormone in your system. So why does TSH go down for some people at this point. Why is the pituitary still not saying “no we still don’t have enough here, come on thyroid, produce more”?

tattybogle
tattybogle in reply to MiniMum97

based on no real scientific knowledge whatsoever.... my gut feel for how TSH works , is that the TSH responds not only to the actual level of thyroid hormones in the blood , but also responds to a change in level ... So even though there is still not enough , it notices an increase happened and responds accordingly by going down(for a brief period?) and then goes back up when it realises it's still not enough.....

I do realise i've just given TSH some properties it definitely doesn't have in this suggested explanation... obviously 'it' doesn't 'know' enough too 'realise' anything :)

Edit*

of course that's all in the comfortable world of the 'theoretical'.... out here in the real world. you have to consider that TSH is up and down each day , and within that to a smaller degree, up and down every half hour.

so unless you measured someone's TSH every half hour over 24hours, a TSH result it's still just a general indication of the area it's in , rather than a specific unchanging value.

(and then you have to include the fact that blood taken from the same arm at the exact same time but processed on 2 different lab platforms will return a slightly different result to each other)

So bearing this in mind ... the answer to your question would really depend on how much has the TSH gone up , or down by.

MiniMum97
MiniMum97 in reply to tattybogle

“Based on no real scientific knowledge whatsoever” 😂😂😂 loving your honesty there 😊

Having said you have no scientific knowledge though what you have said does make some sense to me ie that the reason people feel worse on lower doses could be that the pituitary says “eh up! Hold on a minute! The thyroid’s doing something, I can see more thyroid hormone, I’ll chill out a bit and see what it does?” Then when it realises no more is coming and it’s still not enough it picks up again.

Obviously I also realise that I am also anthropomorphising the pituitary and it does not see, think or indeed speak 😊

Thanks for your replies, your explanation makes sense to me so I’m going to go with this unless anyone comes along with some actual scientific knowledge to tell us we don’t know what we are talking ahout 😊

Thanks again. Good chat 😊

tattybogle
tattybogle in reply to MiniMum97

:) i was just editing that , so it's got more complicated now .... soz

pituitary gland senses increase in T4 in blood.

So pituitary reduces TSH output. (Thyroid Stimulating Hormone is the driver for how much t4 and t3 is released from thyroid gland)

Less TSH = less of your own t4/3 output from thyroid gland. (it won't stop it completely, but it will be less than before)

Less TSH also reduces T4 to T3 conversion via the deiodinase . (so you can end up doubly worse off , less T4 and less T3)

Eshep
Eshep in reply to tattybogle

Thank you!! When you have a moment, can you please check out my most recent post about my thyroid mystery. I could really use some legit insights.

tattybogle
tattybogle in reply to Eshep

i;m not good at understanding iron/ ferritin , i think you need humanbean for that. click on her name , she has lots of posts and replies on the subject.

Eshep
Eshep in reply to tattybogle

Thank you! Anyone else you would recommend who knows like everything there is to know about the thyroid?

Scrumbler
Scrumbler in reply to Eshep

greygoose is very knowledgable

greygoose
greygoose in reply to Scrumbler

But far from knowing EVERYTHING. lol I don't think anyone can claim that. It's a lot to ask. :)

Scrumbler
Scrumbler in reply to greygoose

😎😎. Go on Gg. You can do it!!

greygoose
greygoose in reply to Scrumbler

Replied at length on Eshep's other post. :)

Eshep
Eshep in reply to Scrumbler

Thanks! Already contacted them!

greygoose
greygoose in reply to Eshep

Her. There's only one of me. :D

Eshep
Eshep in reply to greygoose

Well didn’t know if you were a he or a she. So said “them”. Perhaps I should have written she or he.

Eshep
Eshep in reply to Eshep

And you haven’t solved my issue yet. 😁

greygoose
greygoose in reply to Eshep

Well, you haven't given all the relative information.

Eshep
Eshep in reply to greygoose

I’m trying. Will give more once I’m home and look old labs over.

greygoose
greygoose in reply to Eshep

👍

Everyone is different.... I started on a minute dose of thyroid meds and slowly built up over six months. At no point did I feel worst than before.

I'm currently just starting my second lot of block and replace for graves disease and have learnt something I didn't know.Now I know why I feel so awful on 50mcg of thyroxine.I'm waiting for a dose increase after my blood test last Monday.Think I'll chase it up tomorrow.Thankyou all.

SeasideSusie, SlowDragon, tattybogle could one of you point me to the research to support your claims that any endogenous meds shuts off thyroid production please?

tattybogle
tattybogle in reply to Evej13

Personally i wouldn't say that it does shut it off completely. And i would expect 'what happens next ?' to be rather dependent on the capacity of the individual thyroid gland in question to produce more fT4/3 than it was doing in the first place. And also dependant on the particular level of TSH before additional hormone was taken. And on lots of other subtleties. This is a very complex regulating system we are interacting with. But adding T4 should be noticed by the pituitary , which should lower TSH (for how long though, and by how much is a how long is a piece of string type question) and so you would expect less thyroidal production of T4/3 for a short while at least.

edit* .... and if the sum of the now lower thyroidal t4/3 and the added t4 was less than previously, you would then expect TSH to start to rise again in response.

Evej13
Evej13 in reply to tattybogle

That sounds more in keeping with my understanding of a complex negative feedback loop than shutting off production altogether. I would think as long as there is a thyroid that is not completely dead or removed, there should be at least some endogenous hormone produced by the thyroid in response to rising TSH (unless one has central hypothyroidism). Regardless, if there is research to support, I would like to explore that.

tattybogle
tattybogle in reply to Evej13

complex indeed , the more you learn , the more complicated it gets.

Like there being more than one sort of TSH , with different 'strengths' so i assume that means 'higher TSH' may not necessarily mean 'more TSH action' .

And there being variable half life of T3 between hypothyroid/ euthyroid/ hyperthyroid patients. (shorter in hyper).

And there being not only a feedback mechanism but also a feedforward mechanism, deiodinases , i think , but can't remember clearly at this point on a sunday (half past glass of wine o clock)

The thyroid is definitely very complex. Not sure I will ever grasp all the ends and outs.

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