| TSH
level above 2 has important implications
The concept of subclinical hypothyroidism is based on the log-linear
feedback loop between thyroxine and thyroid stimulating hormone:
for one unit change in thyroxine there is a 10 unit change in thyroid
stimulating hormone.
The prediction of disease by the measurement of an intermediate
marker is now well established in the absence of clinical symptoms
for example, calcium concentration, cholesterol concentration, and
blood pressure.
It has been shown
that changes in endothelial function and cholesterol concentration
are apparent in subclinical hypothyroidism and even in people whose
thyroid stimulating hormone is greater than 2 mU/l.
(3,4) This provides a physiological basis for the Rotterdam study,
which clearly shows that subclinical hypothyroidism is associated
with an increased risk of ischaemic heart disease.(5)
A value outside the reference interval is not simply a minor variation
but is important both in terms of predicting future hypothyroidism
and in causing biological effects. (3,4)
Alun
Price, chief medical laboratory scientific officer, clinical
chemistry. A P Weetman, honorary consultant physician. Northern
General Hospital, Sheffield, UK
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