We have exactly the same problem, with fT3 far less raised than expected in
clinically thyrotoxic patients. We suspect that it may be an assay /
reference range problem, and we are investigating this at the moment.
Our assay is Roche Elecsys.
Avril Owen
Biochemist
Ysbyty Gwynedd
Bangor
Wales
-----Original Message-----
From: Samuel Vasikaran [mailto:[log in to unmask]]
Sent: 28 May 1999 02:45
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Re: Testing for hyperthyroidism
Since we changed over to measuring free hormones, our experience with
our free T3 assay has not lived up to our expectations. I have seen
many instances where FT4 was raised with TSH < 0.01 but with a high
normal FT3. Many of them were in clinically thyrotoxic patients. I
now recommend a thyroid uptake scan in these instances if the
diagnosis is in doubt (mainly because of the non-raised FT3). This
may just be a reflection on the particular FT3 assays that we are
using. I wonder if others have had similar experience.
Samuel Vasikaran
Chemical Pathologist
Royal Perth Hospital
> At present we measure TSH alone as an initial test for thyroid dysfunction
> in patients not on treatment. If this is below the reference interval we
> cascade onto fT4 and then onto fT3 if this is >18pmol/L. We also do fT3 on
> lower levels of fT4 if the clinical information suggests hyperthyroidism
is
> to be excluded. In short, much more emphasis is placed on the fT3 (which
> rises earlier and proportionately higher in hyperthyroidism) than fT4.
>
> I know this may sound heretical to some (mainly because of analytical
> considerations), but is there any point in measuring the fT4 in this
> situation? Should we not just cascade from low TSH directly to fT3 (which
> costs the same as fT4) and be done with it?
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