Determination of thyrotropin (TSH) by sensitive immunometric assays is curr
ently judged as the most sensitive and also most cost-effective first-line
approach to thyroid function testing. Further improvement of assay sensitiv
ity has led to the description of third generation TSH assays with a functi
onal sensitivity in the range of 0.01 to 0.02 mU/1. In the present study, w
e analyzed interassay precision profiles of a commercially available third
generation assay (ACS:180 TSH-3) and documented the critical role of the ti
me span used for the assessment of a method's functional sensitivity. By us
ing a standardized approach with five serum pools measured in 30 different
runs across a 6-week period, functional sensitivity was calculated as 0.015
mU/1. The TSH concentrations measured by two different third generation as
says (ACS:180 TSH-3 and Elecsys TSH) in samples from healthy blood donors w
ere highly correlated (r = 0.76, n = 252). In some samples, however, discor
dant results were obtained. Euthyroid reference intervals were determined a
s 0.30-3.68 mU/l for the ACS:180 TSH-3 assay and as 0.36-3.64 mU/l for the
Elecsys TSH assay. Reevaluation of reference intervals including only TPOAb
or TgAb negative samples resulted in almost the same reference ranges. Mea
suring TSH concentrations in various patient populations, third generation
assay turned out to be advantageous in the following clinical situations. (
a) In patients with mildly suppressed but well detectable TSH concentration
s due to functional thyroid autonomy (0.03-0.3 mU/l), overt hyperthyroidism
can be excluded by third generation TSH measurement alone without the need
of additional thyroid hormone measurements; (b) in patients receiving long
term suppressive T4 treatment after thyroidectomy for differentiated thyro
id cancer, measurement of basal TSH by third generation assays allow accura
te monitoring of hormone therapy without the need for TRH testing; (c) in m
ost patients with severe nonthyroidal illnesses and decreased TSH levels, T
SH concentrations measured by third generation assays are only moderately s
uppressed and can be clearly discriminated from undetectable levels in over
t hyperthyroidism. In conclusion, the use of third generation TSH assays is
recommended in specialized clinical laboratories frequently analyzing samp
les taken in one of those clinical situations.