The upper limit of the reference range for thyroid-stimulating hormone should not be confused with a cut-off to define subclinical hypothyroidism

This version was published on 1 March 2009

Ann Clin Biochem 2009;46:93-98
© 2009 Association for Clinical Biochemistry


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Ahmed Waise1,2 and
Hermione C Price2

1 Laboratory Medicine;
2 Centre for Diabetes and Endocrinology, York Hospital, York YO31 8HE, UK

Corresponding author: Ahmed Waise. Email: ahmed.waise{at}

The upper limit of the reference range for serum thyroid-stimulatinghormone (TSH) is used to assist in identifying individuals withhypothyroidism. Improvements in TSH assays have led to betterdefinition of the lower limit of the reference range, but theupper limit of the range for a healthy population is currentlya topic of some debate. Population studies have improved ourunderstanding of the clinical implications of elevated serumTSH concentrations in terms of future progression to hypothyroidism,but have not yet fully elucidated the correlation of modestlyelevated TSH levels with long-term morbidity. This paper willreview the current debate including the arguments for and againstreducing the upper limit of the TSH range, whether such a levelshould be based on evidence from epidemiological studies, andthe implications of categorizing large numbers of people withsubclinical hypothyroidism. The impact of using different methodologiesfor the measurement of TSH and the inherent variability of resultson reference ranges is also discussed. We argue that the referencerange for TSH should be assay-specific and be determined bystandard techniques in normal populations as recommended bythe National Academy of Clinical Biochemistry. In contrast,we suggest that a decision level be determined separately fromepidemiological studies to identify a population with subclinicalhypothyroidism. Serial monitoring of TSH in this populationdeserves further study as a means of identifying those at riskof progressing to frank hypothyroidism and meriting treatment.

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