Month-to-month variations in serum thyrotropin
and thyroxine concentrations in normal subjects
(July 2002)
The background of the study.
The reference ranges for serum thyrotropin (TSH) and thyroxine (T4)
in normal subjects are broad, which helps to explain the high frequency
of subclinical thyroid disease (high or low serum TSH concentrations
and normal serum T4 concentrations). The variations are due to analytical
and biologic variation, which may include circadian and seasonal
variation both within and between subjects. This study was done
to determine the extent of variation within and between normal subjects
over a one-year period.
How the study was done.
The study subjects were 16 normal men living in Denmark. Blood samples
for measurement of serum TSH and T4 were collected between 9 am
and noon monthly for 12 months. The samples were analyzed at the
end of the study. The analytical coefficients of variation of the
assays ranged from 2 to 4 percent.
The results of the study.
There were substantial variations within subjects and even larger
variations between subjects for all measurements, so that thyroid
function was unique in each man. The variation for each man was
approximately half that for the group as a whole, indicating that
an individual man could have substantial changes in serum TSH or
T4 concentrations, yet the values remain within the group or reference
range.
There was a weak positive correlation between serum TSH and T4
concentrations. Based on the analytical and within-subject variations,
highly precise (90 percent accurate) definition of these interrelationships
would require repeated measurements. Based on these same variations,
to be significant at the 5 percent level the measured serum TSH
concentration would need to change on average by 0.75 mU/L, and
the respective change in serum T4 would need to be 2.2 µg/dl
(28 nmol/L).
The conclusions of the study.
Serum TSH and T4 concentrations vary substantially from month to
month in individual normal subjects and even more so between normal
subjects.
The original article.
Andersen S, Pedersen KM, Bruun NH, Laurberg P. Narrow individual
variations in serum T4 and T3 in normal subjects: a clue to the
understanding of subclinical thyroid disease. J Clin Endocrinol
Metab 2002;87:1068-72.
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