Re: Oscillations in Thyroid I-131 levels (Reply)

[Ivor Surveyor <isurveyor@vianet.net.au>]

>
>My interpretation of this is that the thyroid is
>"recharging" itself with thyroid hormones while the cat
>is sleeping, thus increasing the level of I in the
>thyroid.  When the cat is active, these hormones are then
>depleted from the thyroid, causing the lowering in dose.
>
>For you thyroid experts, is this a correct
>interpretation?  Could you provide a more detailed
>explanation?
> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>Reply:

Ingenious explanation but most unlikely.  The thyroid or other ductless 
gland for that matter never "re-charge" themselves in the sense you suggest.

There will be some re_distribution of iodine -131 in the animal.    In 
first few hours from stomach to either thyroid or to kidneys, bladder and 
the exterior.      Over a longer time span  labelled Thyroxine is known to 
accumulate in the liver,  one of many target sites.     From the liver 
fecal excretion is possible.    Generally de-iodinated and iodo tyrosine 
metabolites etc will go from thyroid to plasma to kidney and then to bladder.

Of course we have no details of the measurements or time relationships and 
so on.     My best suggestion by far is that this story is due to 
measurement error.    Careful measurement of thyroid uptake would require a 
specially designed collimated detector, carefully positioned over the 
thyroid and held in such a way as to eliminate either animal or human 
movement.

If the intention of the measurement was use the survey meter as a surrogate 
whole body counter, again you would have geometric considerations, as well 
as a variable distribution of iodine-131 from urine contamination and so on 
to confuse your measurements.

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Ivor Surveyor [isurveyor@vianet.net.au]
Emeritus Consultant Physician
Nuclear Medicine
Royal Perth Hospital

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