Is Thyroxine Dosage Determined by Blood Volume?

Hi

I was told by one of my doctors that the thyroxine dosage is related to the volume of blood in a person.

Therefore, she said, any weight gain was irrelevant as thyroxine works in the blood. Is this accurate?

As I no longer hold anything these previous doctors said and did with much confidence, I'd really appreciated some information from this forum.

Many thanks

Nadine :)

14 Replies

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  • Initial dosage for thyroxine is based on body weight: 1.6 mcg/kg. Then it is fine tuned depending on how a person feels and blood results.

    Hypothyroidism results in a reduction of blood volume. So matching dose to blood volume will guarantee under dosing.

  • There is a paper very recently describing this way of determining T4 dosage and querying a TSH-led method of determining correct dosage:

    Do thyroid-stimulating hormone, body weight, or body mass index serve as adequate markers to guide levothyroxine dose titration?

    Journal of the American College of Surgeons 10/2013; 217(4):752-3.

    This may be on file at HU

  • Can't find it without having to pay online.

    What were the general conclusions?

    Thanks :)

  • The general findings were that 1) Estimating T4 dosage by body mass index or by weight only gave quite different results but 2) estimating dosage by TSH measurement was not good and did not give plausible consistent answers. Overall, the message was to probably choose body mass index (I think that's weight in kg over the square of your height in metres) as this corrects for height whereas body mass alone doesn't. In the end the recommendation is for dosage of 1.6 to 1.9 mcg/kg but not taking TSH values into account as the determinant of proper dosing.

  • diogenes,

    I took several published formulas and put them into a spreadsheet:

    dl.dropboxusercontent.com/u...

    Absolutely agree that you have to start somewhere.

    Rod

  • I know nothing but I will say 'no'. It should be on how the patient feels - if well she's on the right dose, if not, either the dose is too low or too high or the medication doesn't suit her at all and should be offered an alternative.

  • The study as I understand it was to first explore how to estimate the best likely T4 dose, with some reasonable aim in mind to start a regimen of treatment. For many people this is fine ie those who can use T4 only - it gets them quickly to their balance point. Of course, if such treatment doesn't work, then other treatments have to be found, but you have to start somewhere and because T4-only does work for many that is where the start point is. Any competent diagnostician then must respond quickly if a balance point can't be found on T4 alone.

  • Thanks very much. It is a minefield really when you don't get well on T4. As you say, others have no problems.

  • "I was told by one of my doctors that the thyroxine dosage is related to the volume of blood in a person.

    Therefore, she said, any weight gain was irrelevant as thyroxine works in the blood. Is this accurate?"

    Have you considered getting another doctor? And no, that is not accurate. PR

  • Am in the process as the practice kicked me out for making an official complaint against a doctor who made me overtly hypothyroid (when I thought doctors knew best!).

    They offered to still treat me if I dropped the complaint but they were all incompetent and the one who said that was probably one to their better ones.

  • Medscape is usually regarded as a fairly reasonable, if rather establishment, site.

    It has a page for calculating blood volume:

    reference.medscape.com/calc...

    You might note that its only inputs are weight and selecting one of:

    Men (75)

    Women (65)

    Infants (80)

    Neonates (85)

    Premature Neonates (96)

    On the basis that the doctor can't actually directly measure your blood volume, she would have to use some sort of estimate like this. The irony being, obviously, that blood volume would rise with weight. So actually assessing on the basis of blood volume would have some correspondence to assessing on the basis of weight! Or does she have some other miraculous way of finding your blood volume?

    Basing dose on any physical attributes of a person like this will inevitably fail when there are variables like how much levothyroxine is actually absorbed, and batch-to-batch and make-to-make variations.

    Rod

  • Rod, I think we need to add what her doctor said to the 'Doctors say the darnedest things' category. What on earth are they teaching these people in medical school? PR

  • Is it what they are being taught or what they are learning? Not always the same thing. :-)

  • Seen the volume of blood questioned before. A normal result would result!

    Has every adult got 8 pints of blood disregarding their size?

    Vets go by weight, yes, where does it start? J :D

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