Estimating Full Replacement Dose of Levothyroxine: A New Formula

Estimating Full Replacement Dose of Levothyroxine: A New Formula

When someone has a total thyroidectomy they are sent from having a wide range of thyroid levels – some would have been euthyroid, some hyperthyroid, some hypothyroid – to a state of having no source of thyroid hormone – within an hour or two.

This is obviously a very bad situation. Equally obviously, going through the usual dose titration from 25 or 50 mcg upwards over many weeks or months is not desirable. Why should they suffer an extended period of hypothyroidism? Replacement to somewhere near where they were before the operation should be achieved as quickly as possible.

One traditional way of estimating how much they might need was to apply a formula:

1.6 micrograms of levothyroxine per day for each kilogram the person weighs

(You would sometimes see doctors suggesting anything from 1.6 through to 2.3 mcg/kg.) So for someone of 60 kg that would work out at 96 mcg – and they would be given 100 mcg as the closest dose.

It was never a very well researched thing - more a rule of thumb. Many patients ended up needing more, but it did help to estimate a sensible starting dose which would be unlikely to overdose significantly.

More recently another formula was suggested that accounted for age and reduced the requirement as people aged. (I blogged at the time.)

Now, yet another formula! The people behind this new formula recognise that fat people need somewhat less than the old formula suggested; thin people need rather more. So they have factored in the BMI. And came up with this for the daily dose of levothyroxine:

For every kilogram the person weighs, they should be given this many micrograms of levothyroxine per day:

-0.018 × BMI + 2.13

(The old formula was often quoted using the person’s ”lean body mass” – but never gave any indication as to how you could handle any difference between that and their actual body mass.)

Please note: This is only intended to apply to people who have just had a total thyroidectomy. It is not appropriate to use it for other purposes. Mind, I have sometimes used the original formula as a “sanity check” on people’s doses of levothyroxine. If their dose is miles below or above what the formula suggests, maybe there is a problem to be looked at?

A summary of the paper appears here:

scienceindex.com/stories/27...

Abstract to the paper available here:

sciencedirect.com/science/a...

(Full paper requires payment.)

Out of interest, if you have had a total thyroidectomy, and are on a stable dose of levothyroxine only, would you be willing to post a) the dose you are actually on; b) how much the formula suggests you would need?

If you are not comfortable working it out, send me: your height, your weight and your current stable dose by private message. I will work them out and post the results.

FYI: BMI, body mass index is calculated as weight in kilograms divided by the square of height in meters.

Rod

Picture is a random overweight male

Added 18 January 2013 15:55

Excel Spreadsheet

I have created an Excel spreadsheet that is intended to show the results of all three known formulas. You are welcome to download it from here:

dl.dropbox.com/u/318411/Est...

As it is brand new, and not sufficiently tested, please be careful in using it. And I certainly have not tried using it in other spreadsheet programs.

If you find anything wrong or a problem, please let me know asap by private message.

For now, please do not pass it on to others. I like to get more feedback before I would be comfortable with that.

46 Replies

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  • oh no :(

    something we didn't need by the sounds of it!

  • Yes that works.

    My BMI is 23.1 so at would be 1.7142 x 66 = 113.13 mcg and I am stable (ish) on 100/125 mcg after TT.

  • Oh my goodness that made my brain explode! My BMI is 22.4 I weigh 59 kilos my daily dose is 125 I feel fab on this although my docs would prefer me to be on 100 daily. Is this enough info helvella?

  • According to the formula you would need almost 150mcg daily. If you feel great on 125 then stick with it :)

    Show your doctor the paper with the new formula next time he wants to reduce your dose

    !

  • I'm sorry I made a mistake! Brain fog :(

    It's nearer 102. This is only an estimate though and do bear in mind you might not be absorbing all of what you take. Bottom line - if you feel well you are on the right dose :)

  • Oh my goodness no wonder I am still very ill two years later. My doses of Levo seemed to be guess work and when I couldnt tolerate it and refused to take it anymore I was left for 4 months on nothing until I found this website!

  • Really? gosh that sounds like such a lot. Carolyn I stamped my feet like a two year last time I was with my endo and flatly refused to accept the revised dose (based on my suppressed VTSH) told them I was not prepared to allow them to make me sick. Now won't be seeing anyone for a year!

  • It sounds very much like 125mcg is right for you so definitely stand your ground! There have been a few recent papers about how TSH is unreliable for dosing purposes. Helvella has posted them quite recently on one of his blogs so you may want to have a look and print them off as ammunition for your next appointment ;)

  • Well I haven't had a TT so this doesn't really apply to me, but hese are my figures anyway with a BMI of 23.2 :D :

    1.7124 * 61.4 = 105mcg of thyroxine (hope I have calculated that correctly)

    Well I am on 2.25 grains of Armour - which I think contains 85.5mcg of thyroxine and 20.5mcg of T3 so is roughly equivalent to 166.5mcg of thyroxine. I think I am still a bit under-dosed on this and am hoping to increase to 2.5 grains in February. Not sure if you can use it if you are taking T3 in any form??

    Thanks for posting this Rod :)

  • I make yours 156mcg ;) I have bad brain today though!

    (0.018 x 23.2 + 2.13) x 61.4 = 156.4

    Edit: Ignore me. I missed the - sign! I had a 6 year old criticise my handwriting today so you can see how well I am doing, lol! You were right the first time.

  • Thanks Carolyn - sounds like you have had a busy day. I don't know how you teachers cope with 30 lively kids all day. I struggle with my 2 :D xx

  • Yeah, we were being "observed" by the head today so lots of stress! Not to mention the kids being completely hyper due to the weather. I feel like I've just got off a rollercoaster! Now trying to deal with my ?aspergers son who is just being unbelievably pedantic about everything today! Now to find something he will eat...

  • Theoretically anyone with any remnant thyroid could be producing some thyroid hormone.

    And it will always tend to produce a value near the bottom of the likely range. I mean, you might only absorb 75% of what that works out at, so need a bit more (towards a third more if that were literal). But you can't absorb 125%! If you see what I mean?

    If we had a decent, logical way of relating T4 and T3, then maybe it could? But we only have a very rough rule of thumb. I know you are "playing" and it is very useful to do things like that. But I'd be wary of being too serious with T3 in the equation.

  • I still have a theory that before diagnosis my FT3 was right at the top of the range or even above and I was an out-lier (spelling doesn't look right?). I wonder this due to having a high TSH of 18.0 and "normal" ft3 and ft4 (close to the bottom of the range). Time will tell on the correct dose I hope :)

  • The old method of 1.6mcg per kilo would mean 98.2mcg of thyroxine - so this method is slightly more generous :)

  • This looks very interesting. I will have to give it a good read when I can read better than a six year old. Perhaps after a good night's sleep ;)

  • Ok... brain starting to function... stand by...

    If it's going on BMI it still doesn't work that well. Some of us have a higher BMI due to higher muscle mass (I do a lot of karate and strength training) so for some people it would give a result that is on the low side.

    And I bet that didn't even make sense :(

  • But maybe if you have a higher muscle mass and therefore a higher BMI you need the extra thyroid hormones that this will allow for :) Have I got that right?

  • I'm confused :( I thought it predicted a lower amount for those with a higher BMI (hence the - sign) and more for those with a lower BMI. It seems to be basically making the assumption that people with a high BMI will carry more fat. Of course this isn't always the case among those who are very physically fit. Many rugby players have high BMI but very low fat percentage so it would not work for them.

    That's my understanding anyway but, given my track record for today, I could be very wrong ;)

  • Ahh I think you are right - I wondered what the - sign was for. Yes you are right then this wouldn't work for people with a high BMI then :(

  • It would work for them if they were of an average composition but it may well be lower than other formulae would calculate and I'm sure that's not what a lot of larger people want to hear (me included!). I suppose the same would go for skinny people with no muscle tone, except theirs would be over-estimated.

    My personal opinion about all these calculators is that they are good for getting an idea of what sort of dose to aim for but that it should be fine-tuned according to symptoms. Doctors seem to be very autistic about these things... the formula says, so it must be true... your TSH is in range, you must be fine... etc.

    I think we should go by symptoms but that would be far too easy ;)

  • Yes I think you are right :) It looks I am already on a much higher dose than the calculator would predict. Hey ho :D

  • It is saying that an increased BMI is due to fat and fatty tissue needs less thyroid hormone.

    If I were you, I would consider emailing the authors of the paper and pointing that out. Seems to me it might have a good side to it (probably increasing doses for the skinny), the problems of the BMI (being an over-simplistic formulation) will push this new formula from the straight and narrow.

    I have found many people are happy to hear from people who can provide helpful input.

  • I've not had a tt but I weigh 65 kg and am 5ft10 tall. When on just thyroxine I needed a dose between 100 and 125. 10 years ago age 23 it was increased to 125. Tsh increased and switched to 100ug t4 plus 10 t3. Then it was increased further after tsh rose again to 125ug t4 and 10ug t3. So as I've got older I've needed more.

  • You are quite right to make this point.

    I think that the "needing less with age" issue is often meant to apply to people of no relevant economic worth- i.e. retirement age onwards.

    I wonder whether any research has gone into changes in requirement at the ages you mention?

  • Proffeosr Atkin at Hull came up with another formula for prediciting thyroxine requirements following total thyroidectomy.

    (levothyroxine dose= bodyweight - age + 125) . This brings patients into the reference range. NB not when the patient feels well.

  • That was the other formula I referred to.

    I did not include it on my post as such so as to reduce possible confusion. But am very glad that you have done so.

    At the moment, I am feeling that we need to see that one, plus the new one I posted, plus comments (e.g. about higher BMI from muscle rather than fat) all combined in some way.

  • This is really interesting although I must say that I am more interested to see that people who have had a TT are able to maintain a healthy weight.

    I had a TT two years ago and weighed 62kg. I now weigh 78kg and despite having been working out for 1.5 hrs/day, 5 days a week and eating 1500 calories/day for about 9 months last year - I could not shift 1 pound! I have stopped working out, I walk 1 hour to and from work ev. day, eat sensibly, but just keep piling on the pounds!

    Not sure what my BMI is - my height is 5ft 5"; weight 78kg and I take 112mic levo ev. day. Maybe Rod can calculate what my dose should be?

    I must add - I experience dreadful tachichardia most days - it even wakes me up when I am sleeping. I no longer sleep more than 5 hours (thats a good night lol) and I have a digestive system that has almost ground to a halt despite eating high fibre/veg and drinking plenty of water - the lavatory is a painful and bloody experience once every two weeks (apologies if that is too much information)!! Memory foggy - an inability to carry out mundane familar processes/cholesterol soaring (never had cholesterol probs)/eyes bulging and sore/lots of shivering - are everyday symptoms.

    I would love to hear from those who have had a TT and 'feel great' - what they eat, what dose they take, how long since the op. etc. etc. to get some anecdotal stuff that might improve my situation.

    Many thanks in advance, Louisa

  • Hi Louisa I had TT Sept 11 I'm on 125 levo daily (although my docs would prefer me on 100) &uthere isn't anything much I don't cope with. I was on high dose steroids for 18 months and put on 18lbs despite restricting myself to 900 cals a day. Now off those I allow myself 1100 cals daily and run twice daily with the dog and attend 6 to 8 aerobic/zumba classes a week. Its bloody hard work but I'm determined to shift my extra weight whatever the cost! You don't sound to me like you are medicated enough go back to your doc & throw a tantrum - it is your docs responsibility to make you feel well and you should remind them of that!!! I really hope you start to feel better soon xx

  • From the spreadsheet (see updated blog above) - I do not know how old you are, so put in 40! That only affects the Hull value:

    Estimation of Post-Total Thyroidectomy Levothyroxine Requirement

    Your Details

    Height in metres1.65metres

    Weight in kilograms78kilograms

    Age (to nearest year)40years

    Wisconsin Formula

    BMI28.65

    Micrograms per kilogram1.61

    Total micrograms per day125micrograms

    Hull Formula

    Total micrograms per day163micrograms

    Traditional Formula

    Total micrograms per day (at 1.6 mcg per kg)124micrograms

    Total micrograms per day (at 2.3 mcg per kg)179micrograms

  • First method dose 136 (140)

    New method dose 139 (140)

    However, on t3, so do we divide by 4 or 5? Current t3 dose 50 mcg. Last T4 dose before changing to T3, which still left me with symptoms was 150 mcg. Seems like I need more than formula...

    Please note that I haven't had a thyroidectomy but have hashis and no thyroid function left, so thought this may be similar?

    Mary

  • While we have rules of thumb for "converting" T4 to T3 in dosages, they are definitely imprecise.

    Remember, this formula is expressly to provide a best guess for starting without overdosing. Many will need a bit more.

  • The first 2 sentences of the paper Prof Atkins collaborated on states

    Levothyroxine replacement to within the reference range is key to prevent hypothyroid symptoms following total thyroidectomy for non-malignant disease. However, some patients require several dose adjustments to achieve this.

    Predicting thyroxine requirements following total thyroidectomy.

    ncbi.nlm.nih.gov/pubmed/211...

  • By the way, did you notice that I have updated the blog?

  • Reading the above paper closer, there are some interesting passages

    All patients were started empirically on 100 mcg levothyroxine

    and the dose increased every 6 weeks to ensure that the TSH (0.5–

    4.6 mU/l) and serum thyroxine (10–24 pmol/l) were within the

    reference ranges. Patients were asked to take their thyroxine once

    daily in the morning. Patients were reviewed 6 weeks post-operatively

    until their TSH and serum thyroxine were within the reference

    range;

    additional dosage alteration based on symptoms was

    undertaken as part of this study.

    The median levothyroxine dose was 150 mg (standard deviation

    46). The mean TSH was 2.1 mU/l (standard error of mean 0.1),

    and the mean free T4 was 16 pmol/l (standard error of mean 0.3).

    The mean thyroxine dose per kilogram (kg) bodyweight was

    2.00 mcg/kg (standard deviation 0.61). The data of patient age and

    bodyweight were divided into quartiles.

    The mean TSH for the 1st, 2nd, 3rd and 4th age quartiles was 2.0, 2.4, 2.0 and 2.0 mU/l, respectively

    The mean TSH for the 1st, 2nd, 3rd and 4th weight quartiles was 2.3, 2.0, 2.2 and 1.9 mU/l, respectively

    The time taken for patients to achieve optimal levothyroxine

    replacement from the time of their surgery has been found to be

    highly variable (median 14.5 weeks, range 2–120 weeks) and

    dependent on the magnitude of change in dose from baseline

    It looks like (despite all the problems re using TSH as the only marker and noting that in this study additional dosage alteration based on symptoms was undertaken) that optimal TSH for those with no thyroid gland is around 2.1 mU/l.

  • My assumption, after all the calculations, dosing, etc. It still comes down to the medical people diagnosing you on the TSH blood test and pronouncing you euthyroid and not how well/ill you feel.

  • another way to remove the pesky patient from the important work of the doctor.

  • thanks Rob. Which blog?

  • This one!

    Look at the top - the actual blog,

  • Right. Thank you.

  • Good day Rod

    I would like some advice, My thyroid was removed last year April, and from then we are struggeling to get the correct dosage, I lost our baby last month and my gynie said that i am not allowed to get pregnant before my Thyroid function is not working correctly and we are currently stil struggeling to get the correct dosage, I Started om 100 just before they removed my thyroid, and after it was moved up to 150 but was not working, so it was increased to 200 but was to high, now I am on 175 but it is still not the correct dosage? I would also like to know how long does it take for the dosage changes to take affect? How soon would a blood test show the correct results after changing the dosage?

  • I suggest that you are likely to get a better response if you start a new question.

    If I answer here, no-one else is likely to jump in! :-)

    Rod

  • Mine figured 116.- micrograms. Had been doing well at 137 for last 11 years, recently went up to 150 after starting g a routine exercise regimen. I think k they need a formula based off of kilocalories????? I have complete eradication of my thyroid d/t radioactive iodine. Would love to hear your thoughts.

  • I don't think that a formula has any place once someone is on a steady dose of thyroid hormone.

    The only possible use is to do a "sanity check" - is the person on a ridiculously low or high dose? Even then, if they are on a very high dose, it might still be what they need.

    Furthermore, as you suggest, variations in an individual from day to day, week to week, year to year, are important. Whether these are due to exercise, weight, other illness, their diet, or anything else. It is common enough for people to feel they need a small increase during the winter.

  • This is interesting. Just a question on thyroid in general - I've been on T4 for 20 years - never had any part of thyroid removed but many years in they did do a scan and I think the results were that thyroid was indicative of post-treatment condition. I've been on 150mcg T4 for over 10 years - appear not to be converting well to T3 based on labs and have had issues with sleep, agitation, etc. which I think result from having too high a T4 as Free T4 is always slightly above or at top of T4 range (Free T3 is at about a third of range). Does a thyroid still function even after 20 years of treatment? I read somewhere in the past that once you get to a full replacement dose it really takes over and your thyroid slowly stops producing its own. So if that is the case, based on the figures in the spreadsheet and my particulars, I think I should be on a lower dose. If the thyroid DOES still have some function I certainly should be on a lower dose based on my particulars (76 kgs, 1.72 meters, male 42 years old). I am considering dropping my T4 med down to 125 based on what I see here and see if that helps me symptom-wise and then perhaps work on boosting T3 otherwise. Any thoughts? Great resource this is! Thank you.

  • I have just noticed that you posted the above question 3 months ago and don't know whether you had any 'messages' i.e. private between you and a member. However, seeing that the original post is four years old, your question might well have been overlooked.

    I hope you posted a new question on the main forum and got a response.

    :)

    By the way you should have a Free T4 and Free T3 test as that would indicate if you needed more or less hormones, the FT3 in particular as T3 is the only active hormone which drives our metabolism and makes us feel well. T4 (levothyroxine) has to be converted to T3 and we don't always have sufficient to make us symptom free.

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