Dear Gary,
I am wondering whether TBG measurement fulfils more than just a
curiosity. If I remember correctly from the old book 'Thyroid' (sorry; I
can't remember the author/editor), TBG-deficient patients have a normal
FT4, and, perhaps surprisingly, don't even have a goitre. I can understand
that although TBG carries 70% or so of T4 in circulation, in its absence
the amounts of T4 carried by TBPA and albumin increase dramatically even if
their concentrations remain the same - since TBG, by virtue of its higher
affinity for T4, normally displaces the hormone from both other BPs. And,
although I am probably wrong, perhaps the interest of Prof. Ekins in a
TBG-deficient female and her off-spring stems from a quest to understand
what role TBG plays in carrying thyroid hormones in the fetus and
importantly across the placenta.
With your first-hand experience of the TBG assay and the type of patients
that are referred to you, I'd be most grateful if you'd let us know in what
situations TBG measurement is indicated. Do you re-measure FT4 in the
samples that you receive, and how often the reason for the referral is a
problem with the original FT4 assay? I stress that by no means I disregard
the value of TBG assay in furthering our understanding of thyroid hormone
metabolism.
With best regards,
Reza Morovat
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