Dosing Thyroid Meds by BMI Post-thyroidectomy - Thyroid UK

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Dosing Thyroid Meds by BMI Post-thyroidectomy

I found an interesting study that discusses dosing thyroidectomy patients according to BMI rather than weight (endocrinologists often start thyroidectomy patients at doses equivalent to their weight, so a 125lb woman would get 125mcg, etc).

The formula they propose is this:

Mcg/kg/day = .018 x BMI + 2.13

When I did the math for myself, the result was higher than any dose of thyroid hormone I've ever been given, which maybe explains why I began to develop symptoms in the years after my thyroidectomy.

I tried the formula at two different weights my body tends to gravitate towards, and these were my results:

BMI 21.5 = 142.7 mcg

BMI 20.6 = 136.1 mcg

I'm on some NDT now, but the most levothyroxine I was ever given was 125mcg, and then had my dose slowly lowered until my TSH was "normal" at 100mcg (which in retrospect was ridiculous, because I didn't have a thyroid for the TSH to stimulate!)

Anyway, I thought this might be interesting for other TT members out there. What do you guys think?

12 Replies

Some time ago I created a spreadsheet which used several different formulas to estimate post-thyroidectomy levothyroxine requirements. You might need to download it and open in a spreadsheet program such as Excel or Open Office (or similar).

I do not believe in dosing by weight or BMI.

There is some logic to considering weight (body mass) on the basis that all tissues require thyroid hormone and there will be some minimum requirement “per kilogram”.

Once you add in factors such as how well, or poorly, people absorb levothyroxine (something that varies between people and, quite feasibly, from time to time within an individual) it is obvious that requirements could well be higher than calculated, and variable.

But on what basis can you argue for using BMI – which is based on weight and height? Possibly, if you assessed body composition (some percentage bone, some fat, some muscle, etc.) and had a levothyroxine requirement by tissue type, you could make a case. I don’t believe that has been done.

Having said that, I do believe calculation can give a pointer, a start point. Once started, though, blood tests (I’d hope primarily FT4 and FT3 – but I know that is unlikely) and how people feel are more relevant.



I have not had my thyroid removed as I did not have one to start with except for a (partial non working gland) so been on medication all my life.

I do not believe in the BMI as I think it is misleading and outdated personally. All I know is, that I am happy and function normally on 200 mcg of T4 and 20 mcg of T3.

One GP did dramatically reduce my medication when I was in my 30's and it caused a lot of health problems and heartache, some I am still recovering from, including dramatic weight gain. But I am able to lose weight and I do not get the brain fog or fatique I used to have, amongst other symptoms.

I personally, believe you are likely to be under medicated. Our TSH will never be "normal" and it is not an accurate test. My TSH levels have ranged from 0.09 and now 4.84?

Anyone without a thyroid in my humble opinion, should be anywhere between 125 mcg upwards on T4. I most probably would not have needed the T3 if they had left me alone initially, as I was fine on the T4 without any problems.

I am not sure how the NDT works, but hope you are well on it, if not please get a second opinion.

Take care :)

1 like

Interesting! As a thyroidless person too I have been on many different doses over the years of different types of thyroid replacements.

Like you I’m doing well now on ndt with some T3 , but will do the sums to see if it’s anywhere near the dose I’m on presently , thanks for posting 👍😊


I never in 3yrs after thyroidectomy had one Endocrinologist dose my thyroid meds based on my weight or BMI, I currently take a dose for someone who weighs 100 pounds and no where near my weight.

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Interesting a lot of people are saying they needed a dose much higher than you mention. I have some resistance issues, so 6.5 years after my thyroidectomy I take a huge amount more than that.

I agree with others that these estimates are a bit of a red herring. The initial dose after thyroidectomy should be adjusted based on blood tests and symptoms to an optimal dose for they patient.

The first few months after a thyroidectomy will always be very stressful on the body, though :( If the intention of the estimate was entirely geared towards reducing that stress I think it would be a good thing (although you'd almost certainly include some T3 and probably NDT for that goal, to replicate as closely as possible the natural supply that has been lost). But I think the intention of these estimates is often to send the patient off into the world with only minimal adjustments later. Then they make the argument that because a scientific estimate has been determined that we can't possibly still be sick!


Exactly! I think using some method like this would be better for determining a STARTING DOSE, and then obviously tinkering according to bloods and symptoms, but maybe having to do less tinkering overall than if initial dose was determined by weight alone.

In any case, it’s made me rethink the ballpark amount that I should shoot for. I always though that ~125 mcg or equivalent with T3 as my ceiling, but now I’m thinking maybe more would be appropriate as I still have some bothersome symptoms.


My emphasis

I'm on some NDT now, but the most levothyroxine I was ever given was 125mcg, and then had my dose slowly lowered until my TSH was "normal" at 100mcg (which in retrospect was ridiculous, because I didn't have a thyroid for the TSH to stimulate!)

Were you ever hyperthyroid before your thyroidectomy? Insisting that your TSH must be kept within the reference range when you have no thyroid is sadistic, and its sadistic for lots of other reasons as well. There are lots of people, for lots of different reasons, who have levels of thyroid hormone which are far too low for good health when TSH is in the reference range. Having been hyperthyroid is just one of those reasons.


Unfortunately my doctors didn't do any thyroid tests before the TT (apparently they weren't interested in getting a fuller picture, and it never occurred to me to question them), so I'm not sure what was going on thyroid–wise at the time. I was never diagnosed hyper or hypo in the years before that either.

Hard to speculate otherwise. I had a crazy metabolism as I could eat incredible amounts of sugar without gaining any weight, but I was also very sleepy and took a long nap every day, possibly because I was lazy and full of sugar. :D


I can't fathom why endocrinologists use TSH as a standard for euthroidism in thyroidectomy patients. It just seems... silly. Like a failure of reasoning.

I wish I could understand why doctors treat us the way they do.


Going off-topic...

I noticed you wrote a post about twitching some time ago :

Did you ever fix the problem? Large doses of B12, such as you would get via injection may reduce potassium levels - and low potassium can give you twitching, restless legs and cramp.

Eating a potassium-rich diet should help. If you were desperate it is possible to take potassium supplements but you would have to be careful you didn't overdo it. It can affect the heart rhythm.


Hi humanbean,

That's so kind of you to check in! I still twitch a bit at rest sometimes, but not nearly so often. I think time and extra potassium did help with that, as well as trying not to worry so much. :)


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