Thyroid UK

Body Size Medication?

Does body size play a part in the optimum level of thyroid medication required in each individual do people think?

Is just with a lot of medication it's worked out on body weight, especially something like an anaesthetic. It seems logical to me that the thyroid also varies in size that everyone should be treated on a case by case basis scenario.

As I understand it, the ranges used in thyroid tests don't account for the age of patients or size of patients.

4 Replies

There are many formulas which include body mass and other factors. The common feature is that they really only have two uses.

First: To make an initial estimate as to how much levothyroxine someone might need after a total thyroidectomy.

Second: To make a very rough assessment of whether someone's dose is surprisingly low or high.

Once you are actually taking levothyroxine regularly, then the combination of how well you are and blood tests are more appropriate. Remember that there is significant variation in absorption of levothyroxine between makes, and between people. That alone can make more difference than quite a lot of kilograms of body mass.

For interest, have a look at the spreadsheet I have created from formulas published by various medics:

Please read the limits and warnings carefully.


1 like

As Rod has said, initial dosage can be calculated according to age and size and individual needs vary. However, I have noticed over the years that when I lost weight my TSH went down and FT4 went up and if I put weight back on it went the other way. Having maintained weight loss I needed to reduce dose. This does suggest to me that there is a link though lots of things affect absorption.


I know of someone on quite a high dose of thyroid medication - levo and t3 and she lost 8.5 stone about 4 years ago and hasn't needed to lower her dose of medication.



Thanks everyone, just putting a letter together to an Endo, probably won't make a blind bit of difference but will try and get my point across, if anything it will just get it off my chest.


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