Levothyroxine dosing - micrograms per square metre

[helvella]

I keep finding references to dosing levothyroxine based on body surface area. Such as this:

The maintenance dose is usually 100 to 150 micrograms per m² of body surface.

(Taken from the documentatian for Orifarm but similar is widely included in Patient Information Leaflets, etc.)

And there are body surface area calculators such as this one:

calculator.net/body-surface...

(Chosen as it calculates according to multiple formulas which emphasises the variability of this approach. Which should be taken as a warning not to just take the first number we see.)

A couple of questions:

Can anyone properly and fully explain why this seems to be regarded as an acceptable way of estimating levothyroxine requirement?

Is anyone aware of their dose being calculated like this - and does it work for them?

For me, it calculates a massive overdose!

diogenes

[FarmerDJ]

I was never calculated like this. I was put straight onto 100mcg then 1 increase of 25mcg as it fell in the 'acceptable ' range. Would be interesting to see what it would be calculated in body mass ...

[helvella in reply to FarmerDJ]

I think that is what happened to many of us.

I'm well aware of weight/height/BMI types of calculation. (And very sceptical of their use given the wide range of formulas that produce different results.)

helvella - Estimation of Levothyroxine Requirement in Adults

A discussion about the use of formulas to estimate levothyroxine dosing.

From Dropbox:

dropbox.com/s/gzeknihf815rb...

From Google Drive:

drive.google.com/file/d/1Zs...

A spreadsheet with several active formulas which work out possible levothyroxine requirements including NICE NG145 guidelines. These formulas are referred to in the document above.

This is an Excel spreadsheet but is likely to work in other spreadsheet software.

From Dropbox:

dropbox.com/s/aciho6m7x6f1e...

From Google Drive:

docs.google.com/spreadsheet...

[SeasideSusie Remembering]

So if my body surface area comes out at between 1.66-1.78sq m, depending on whose formula is used, then this works out at

166 - 249mcg for the lower surface area

178 - 267mcg for the higher suface area

or anywhere between 166 and 267mcg Levo.

Well, like you, that would be far too high a dose for me at the higher end, at the lower end it might be OK.

My dosage was never calculated this way.

Seems a very strange way to calculate dosage to me, why not just listen to the patient and take symptoms into account - surely that's the only proper way to do it 🤷‍♀️

[helvella in reply to SeasideSusie]

I think you know how much I question all formulas!

They might suggest an initial dose. They might suggest a gross under- or over-dose. Or identify that the person is needing an unusually low or high dose (quite possibly that being right for the patient).

But I very much agree - firstly listen to the patient - and then test Free T3 and Free T4.

[TSH110 in reply to helvella]

It should be adopted doctors would be really chuffed that their hypothyroid patients were demanding dose decreases , but knowing them they’d refuse to allow it! Mine was done on weight - it was rubbish - but so were any increases (not that I went very much higher than 125mcg cos I was wired) or decreases Levothyroxine just did not cut the mustard for me.

Interestingly my NDT dose was 1.25 grains so close in equivale e to the Levothyroxine dose I felt least ghastly on and very slightly more that the weight formula gave 125mcg vs 125/120mcg alternating days

[TSH110 in reply to SeasideSusie]

In our dreams they might actually ask us how we feel and take note of it! At the end of the day what else really matters or what’s the point of ‘being optimised’ if it’s ignored. Only the opinion of person being treated can determine it, not a load of numbers and formulae. They are not treating a darn spreadsheet of results! But you do have to wonder sometimes as their eyes glaze over staring at a computer screen as if the patient is somehow inside the computer and you are just their avatar sitting in a seat in their consulting room.

I totally agree with you

[jgelliss in reply to SeasideSusie]

Ditto.

[tattybogle]

Would give me far too much ....

This calculation suggests my dose is roughly between 170mcg and 255mcg/day

My weight /height hasn't changed much at all since i started on Levo , but my dose requirement has changed significantly.

I used to have 150mcg /day when i was younger, pre menopause, and very physically active .

I was ok on this dose for about 15 yrs (albeit with TSH 0.05ish and fT4 between 60-130%)

It then became too much and i became clearly overmedicated post menopause.

I now have 112.5mcg/day. which seems about right .. perhaps a smidge too low , but not by much.

[helvella in reply to tattybogle]

We end up that their formulas often (would) overdose us, but their use of TSH all too often underdoses us!

[tattybogle in reply to helvella]

a lot of time/ effort / expense is spent on trying to prove formula's to estimate the correct dose .... i'm pretty sure that if they spent half that time/effort /expense on training Doctors thoroughly in physical signs and symptoms and 'asking the patient the right questions , and listening to the answers' and considering fT4/ fT3 / TSH levels together rather than TSH alone .. they would get to everyone's optimal dose in a much more efficient way than any formula could ever achieve. I can see the attraction of finding a formula that saves time, given how many millions of thyroid patients there are .... but they already know that peoples thyroid levels are by their nature 'individual' ,then why not accept the most efficient way to make a bespoke solution that fits the patient properly is to just get on with it as fast as possible by adjusting it in a 'bespoke' fashion for each individual .

imagine if someone tried to make a formula to simplify buying 'tyres' for all cars... much better to just ask the car owner what size the wheel is , and whether they need to go off road or not .

[TSH110 in reply to tattybogle]

I am not a number!!

[Charlie-Farley in reply to tattybogle]

On point! But formulas are there to try to take the thinking out if the equation!

🤣👍

[TSH110 in reply to helvella]

We could do with a happy medium

[TSH110 in reply to tattybogle]

I’ve found it the other way on NDT I need 0.75 grains more than I did initially…wish it would drop down again

[LindaC]

Seems with so little else ['believed' - WHO in science 'believes'!] they dive on the latest accepted bandwagon...

[TSH110 in reply to LindaC]

I wish that were diogenes’ bandwagon we’d be in clover!

[LindaC in reply to TSH110]

Absolutely! 💚❤️💛

[TSH110]

169- 263 mcg for me….there was clearly room for a huge dose increase just to get to the lowest value using this formula! Maybe that’s why I felt like pants on 120/125mcg alternate days! I only went up to 150 and didn’t like it there maybe another 20 mcg would have done the trick. Interestingly a relative on a suppressive dose - due to thyroid cancer- (tiny compared to me) was on 175mcg maybe there’s something in this formula!

[RedApple]

micrograms per square metre!

How much more dehumanising can they make it? Just let me figure out the amount that makes me feel well and stop trying to fit me into a mathematical calculation!

[helvella in reply to RedApple]

Turns you into a carpet - then they can tread all over you?

[RedApple in reply to helvella]

Very good 🤣, but so close to the truth 😠

[TSH110 in reply to helvella]

🤣🤣🤣….if I were that carpet, I’d be a magic one and fly away!

I have to admit I had no idea I was 1.69 to 1.75 m2 in surface area - an area rug really rather than a carpet but a magic one nevertheless 🤣🤣🤣

“Thyroid patients come in all shapes and sizes, and remarkably, surface areas”

[Charlie-Farley]

Wooo hoo! 🤪👍

Any where between 200 and 300 for me ! I’m on 150 µg of levothyroxine at present. Honestly I do feel ‘ok’ on that dose as all my symptoms have fallen away, but I do miss my ‘overdrive’, that little bit extra I could pull out the hat when things got frantic. Now I just have to pace myself.

I’m getting a monitor my health as I’ve been on this dose almost 10 months. Will be interesting to see where I have settled. Room for a tweak? 🤔. Who knows. 😊

[jgelliss]

It's more Art Than Science.

[kellyjoan]

I am with out a thyroid. I’ve never been dosed by weight but, coincidence or not, my ideal dose is 1.6 mcg per kg.

[kellyjoan]

This may sound inane but do euthyroid people in good health have a predetermined release of thyroid hormones. I think not!

What they do have is a thyroid which will provide flexible and variable amounts of hormone to meet varying circumstances. Hot weather, cold weather, illness ……

In last week’s very hot weather I reduced my thyroid meds and felt much more comfortable. Doing this is quite usual in hot countries

[helvella in reply to kellyjoan]

One of the regular topics here is whether exercise increases rate at which T3 is used - resulting in the need for a slightly higher dose.

[kellyjoan]

So if a euthyroid person in good health exercises, their T4 to FT3 conversion will increase? I suppose cortisol plays a part in this

[helvella in reply to kellyjoan]

I wouldn't like to comment on what actually happens. Maybe the thyroid makes more T3? Maybe both increased making and increased conversion?

But it seems entirely reasonable to suppose that more T3 is made available to cells in periods of exercise.

[TaraJR]

NICE 2019 guidelineNG145 recommends starting to dose by weight. Seems an easier way than trying to calculate than square metres of body!

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.

1.3.7 Consider starting levothyroxine at a dosage of 25 to 50 micrograms per day with titration for adults aged 65 and over and adults with a history of cardiovascular disease.

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

[helvella in reply to TaraJR]

The area approach seems even more bizarre given that the area calculators approximate from actual height and weight. Actual measurement is far too difficult!

Some of the dose calculating algorithm themselves use height and weight. Why we have to divert from just height and weight to consider area, I don't understand!