Thyrotropin/Thyroid Stimulating Hormone (TSH) Measurement

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Specificity

TSH is a heterogeneous molecule and different TSH isoforms circulate in the blood and are present in the pituitary extracts used for assay standardization (Medical Research Council (MRC) 80/558). In the future, recombinant human TSH (rhTSH) preparations might be used as primary standards for standardizing TSH immunoassays.[198] Current TSH IMA methods use TSH monoclonal antibodies that virtually eliminate cross-reactivity with other glycoprotein hormones. These methods however, may detect epitopes of abnormal TSH isoforms secreted by some euthyroid individuals, as well as some patients with abnormal pituitary conditions. For example, patients with central hypothyroidism caused by pituitary or hypothalamic dysfunction, secrete TSH isoforms with abnormal glycosylation and reduced biological activity. These isoforms are measured as paradoxically normal or even elevated serum TSH concentrations by most methods.[195,197,199] Likewise, paradoxically normal serum TSH levels may be seen in patients with hyperthyroidism due to a TSH-secreting pituitary tumors, that appear to secrete TSH isoforms with enhanced biologic activity.[196,200,201]

Technical problems, especially with the washing step, may result in falsely high TSH values.[202] Additionally, any interfering substance in the specimen (eg heterophilic antibodies, HAMA) that produces a high background or a false bridge between the capture and signal antibodies will create a high signal on the solid support that will be read out as a falsely high result [see Section-2C3].[203,202]

The conventional laboratory approach to verifying an analyte concentration such as dilution may not always detect an interference problem. Since methods vary in their susceptibility to most interfering substances, the most practical way to test for interference is to measure the TSH concentration in the specimen using a different manufacturer's method, and to check for a significant discordance between the TSH values. When the variability of TSH measurements made on the same specimen with different methods exceeds expectations (>50% difference), interference may be present. Biologic checks may also be useful to verify an unexpected result. Inappropriately low TSH values could be checked by a TRH-stimulation test, which is expected to elevate TSH more than 2-fold (≥4.0 mIU/L increment) in normal individuals.[204] In cases where TSH appears inappropriately elevated, a thyroid hormone suppression test (1mg L-T4 or 200µg L-T3, po) would be expected to suppress serum TSH more than 90% by 48 hours in normal individuals.

Guideline 18. Investigation of Discordant Serum TSH Values in Ambulatory Patients

A discordant TSH result in an ambulatory patient with stable thyroid status may be a technical error. Specificity loss can result from laboratory error, interfering substances (i.e. heterophilic antibodies), or the presence of an unusual TSH isoform (see Guideline 7 and Table 1). Physicians can request that their laboratory perform the following checks:

  • Confirm specimen identity (i.e. have laboratory check for a switched specimen in the run).

  • When TSH is unexpectedly high, ask the laboratory to re-measure the specimen diluted, preferably in thyrotoxic serum, to check for parallelism.

  • Request that the laboratory analyze the specimen by a different manufacturer's method (send to a different laboratory if necessary). If the between-method variability for a sample is > 50%, an interfering substance may be present.

  • Once a technical problem has been excluded, biologic checks may be useful:

    • Use a TRH stimulation test for investigating a discordant low TSH result, expect a 2-fold (≥4.0 mIU/L increment) response in TSH in normal individuals.

    • Use a thyroid hormone suppression test to verify a discordant high TSH level. Normal response to 1mg of L-T4 or 200µg L-T3 administered p.o. is a suppression in serum TSH of more than 90% by 48 hours.

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