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Thyroid RT3 and T3 and tissue signaling

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by , 03-05-2013 at 12:26 AM (1591 Views)

This study of 403 men investigated the association between TSH, T4, free T4, T3,
TBG and reverse T3 (rT3) and parameters of physical functioning. This study
demonstrates that TSH and/or T4 levels are poor indicators of tissue thyroid levels
and thus, in a large percentage of patients, cannot be used to determine whether
a person is euthyroid (normal thyroid levels) at the tissue level. In fact, T4 levels
had a negative correlation with tissue thyroid levels (higher T4 levels were associ
ated with decreased peripheral conversion of T4, low T3 levels and high rT3). This
study demonstrates that rT3 inversely correlates with physical performance scores
and that the T3/rT3 ratio is currently the best indicator of tissue levels of thyroid.
This study showed that increased T4 and RT3 levels and decreased T3 levels are
associated with hypothyroidism at the tissue level with diminished physicial func
tioning and the presence of a catabolic state (breakdown of the body). This study
adds to the mounting evidence that giving T4 preparations such as Synthroid and
Levoxyl are inadequate for restoring tissue euthyroidism and that a normal TSH
cannot be relied upon as as an indication of euthyroidism, as it has a very low
sensitivity and specificity for hypothyroidism. This poor sensitivity and specificity
is further decreased with the presence of one or more systemic illnesses, including
diabetes, heart disease, hypertension, systemic inflammation, asthma, CFS, fibro
myalgia, rheumatoid arthritis, lupus, insulin resistance, obesity, chronic stress and
almost any other systemic illness.
Low T3 syndrome, with low T3 and high reverse T3, is almost always missed when
using standard thyroid function tests, as the T3 level is often in the low normal
range and reverse T3 is the high normal range, again making the T3/rT3 ratio the
most useful marker for tissue hypothyroidism and as a marker of diminished cel
lular functioning. The authors of this study conclude, S
ubjects with low T3 and
high reverse T3 had the lowest PPS [PPS is a scoring system that takes into account
normal activities of daily living and is a measure of physical and mental function
ing]...Furthermore, subjects with high reverse T3 concentrations had worse physical
performance scores and lower grip strength. These high rT3 levels were accompanied
by high FT4 levels (within the normal range)...These changes in thyroid hormone
concentrations may be explained by a decrease in peripheral thyroid hormone me
tabolism... Increasing rT3 levels could then represent a catabolic state, eventually
proceeding an overt low T3 syndrome.
This study demonstrates that TSH and T4 levels are poor measures of tissue
thyroid levels, TSH and T4 levels should not be relied upon to determine the
tissue thyroid levels and that the best estimate of the tissue thyroid effect is the
rT3 level and the T3/rT3 ratio.