Thank you for all that have replied so far (either directly to me or through
the mailbase) concerning my enquiry as to whether we should only cascade to
measure fT3 following a low TSH rather than taking the more tortuous fT4 and
then maybe fT3 route.
Not surprisingly, the responses have fallen into 2 camps. The first
considers fT3 inappropriate as the second line test because of analytical
considerations and missed cases where the fT3 was relatively normal in toxic
patients. The second camp already use the TSH to fT3 cascade and seem to be
happy with it for reasons which include the detection of T3 toxicosis.
Interestingly, most of the labs who take this route and find it satisfactory
use either the Abbott IMX or Axsym fT3 methods. We also use the Axsym fT3
method and find it fairly robust although, as one responder pointed out, fT3
reference intervals are more age dependent than fT4.
Could it be that the clinical utility of the fT3 assay is dependent on the
manufacturer producing it?
Dr. Eric S. Kilpatrick
Consultant in Chemical Pathology
Department of Clinical Biochemistry
Hull Royal Infirmary
Anlaby Road
Hull HU3 2JZ
Tel 01482-674312
Fax 01482-674310
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