Calculating Levothyroxine dosage by weight

[lucylocks]

Hi

I have just read a post were a members G.P. has now agreed to prescribe her levo dosage on their weight.

Can anyone please advise how this is worked out.

Many thanks browny

[MCMC]

Hi Browny,

When I take my dogs to the vets they always weigh them first before prescribing any medication to calculate how much medication they will need.

I think GPs maybe do this without telling us? I'm intrigued by this now and plan to look up Levothyroxine in the BNF, (British National Formulary, the book doctors prescribe from).

Mc

[lucylocks in reply to MCMC]

I will have to look at it also

[Ansteynomad]

I believe it's something like 1.6mcg per kilo of body weight, but I thought this only really applied if you didn't have a thyroid?

[lucylocks in reply to Ansteynomad]

Thanks for replying

[Marz]

My dog is on 400mcg of T4 based on her weight.....perhaps doggies have better treatment. I thought it was 10mcg per kilo.....for doggies that is

[lucylocks in reply to Marz]

I think you are maybe right about doggies having better treatment.

[Marz in reply to lucylocks]

[helvella Administrator]

There are several formulas which were intended to be used for when someone has a total thyroidectomy - simply to provide a reasonable estimate (or maybe "guess") as to how much levothyroxine a person might require. The idea being to give them as much as you can safely, without overdoing it. Then the dose can be adjusted (usually upwards) as needed and in response to blood tests, clinical judgement, etc.

I believe these formulas can have a second function, that of deciding if someone is on an unusually high or low dose of levothyroxine. Not so that the dose can be changed, but simply to identify if that is the case. Often it will simply be what is needed.

Anyway, there are several formulas, and I have put some together in a spreadsheet you can download from here:

dl.dropboxusercontent.com/u...

It will run in Excel and, probably, several other spreadsheet programs.

Don't go mad with what it comes up with - they are just formulas and can be wrong.

Rod

[lucylocks in reply to helvella]

Many thanks Rod, does this only apply then to people with no thyroid.

Does dosing on weight not apply to people who are hypo with a thyroid

[helvella Administrator in reply to lucylocks]

It only applies to people with no real thyroid function, and even then, only really straight after a total thyroidectomy. That is, as a full replacement dose.

Any other state mean that there should be other ways of assessing need - such as blood tests and clinical presentation.

It is ONLY a rough estimate.

We can think of all sorts of reasons people might need much more - for example, if you consume levothyroxine with your breakfast, you have resistance to thyroid hormone, your gut doesn't absorb levothyroxine well, and various other factors.

Rod

[lucylocks in reply to helvella]

Many thanks

[Thyroidmeg]

I've had a total thyroidectomy about 5 years ago I was on 100 on leaving hospital and doctor called me after blood test to say take 200 a day. I was not overweight then . Several stone later I'm on same dose because as long as tsh ok doctor won't change it . So it must not always be based on weight or these 4 stone would have increased my dose surely