----- Original Message -----
From: "Skipper Beers" <[log in to unmask]>
> For a person on a normal amount of thyroid replacement does the
elevated rT3
> show up, or is the measure approximately as good as a TSH reading?
Reverse T3 is a compensating system. It is high in hyperthyroidism and
in diseased states and in a variant of "Sick Euthyroid Syndrome". This
pathway is a catch-basin in order to divert excess T4 to prevent further
body wasting in illness. Reverse T3 accounts for 40% of T4 disposal.
Reverse T3 is produced mainly from monodeiodination of the thyroxine
molecule -- similar to conversion of T4 to T3 but deiodination from a
different location. Alford RA in "Low Metabolism or Hypothyroidism"
observes an anomalous level of rT3 despite adequate to elevated T4
levels in some hypothyroids and has termed it "hyperthyroxinemia
hypothyroidism" -- meaning hypothyroidism at the T3 level due to the
anomalous production of rT3 from T4. During the early stages of
hypothyroidism, T3 production even becomes more efficient because of
compensating Deiodinating Enzyme System in the pituitary and the
brain -- eventually this succumbs to the loss of the gland. His
observations fits the model under which T4 disposal is understood
currently. Alford considers the use of "T3 Resin Uptake (preferrably by
use of Abbott kit) and the "Total T4" as a good approximation of the
presence of excessive rT3 production. T3RU and TT4 may be considered to
be more "real-time" because the values respond fast to varying dose
changes as compared to TSH which takes weeks to re-set. These two tests
are good also for dose adjustment in anti-thyroid treatment becaue of
their fast response.
Suggested Reading: Principles of Internal Medicine, Fauci et al 14th Ed
Vol 2 Diseases of Thyroid
Hope this helps
Greg
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