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Reference values of thyroid function tests are
usually obtained by investigating serum hormone concentrations of
a number of apparently healthy people. The reference range is
then determined using standard methods (like the common
percentile and the log-power gaussian transformation) to estimate
the 2.5th and 97.5th percentiles of the data set. Components of
the observed variation of thyroid function tests are analytical,
intra-individual and inter-individual in nature. Analytical
variation contributes on average 10% to
the total variation, intra-individual variation 20%, and
inter-individual variation 70%. Indeed repeated measurements of
thyroid function tests in the same individual over a period of
two weeks demonstrate a rather narrow fluctuation around the mean
value (9). Apparently each individual is characterized by a
rather fixed relationship between his or her serum TSH and FT4
concentration (Figure 1). This point can be called the working
point of the pituitary-thyroid axis of that individual.
View figure 1
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