Thanks to Geoff Holder for this comment, which I just received;
"The CBG comment was based on the fact that if she has a genetically
determined low level of CBG then her biologically active free cortisol
will be "normal" even with an apparent low total serum cortisol. This will
also have an effect on the total serum cortisol method that uses blocking
agents to push cortisol off CBG so that it can be measured by the
immunoassay. If the CBG concentration is outside the levels that the assay
designers used to calculate the blocking agent concentration then your
total serum cortisol assay may not give the "correct" answer. This would
also explain the very large ratio of basal to stimulated total cortisol
after Synacthen because patients with normal CBG concentration saturate
this at around 650 nmol/L. The fact that the CBG does not change during an
SST is a red herring and not related to my suggestion."
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