Clinical significance of a low concentration of thyrotropin: five immunometric "kit" assays compared.

Abstract

We compared results of five sensitive immunometric assays of serum thyrotropin (TSH) in controls and in different groups of patients with hyperthyroidism, untreated or treated; secondary hypothyroidism; nonthyroidal illness (NTI); or depression; or who were being treated with amiodarone. With most kits, measured TSH concentrations did not overlap between controls and hyperthyroid patients. In untreated secondary hypothyroidism TSH was not always undetectable. Patients with NTI and depression showed many low TSH values, and among these categories of patients, we observed large discrepancies among the kits. This lack of specificity at low concentration means that one cannot assess hyperthyroidism by TSH measurement alone, but it can be used as the first screening test. Similarly, TSH determination cannot be used alone in monitoring therapy (e.g., with carbimazole, thyroxin, amiodarone) to assess the presence of hyperthyroidism. Nonetheless, this assay plays a well-established role in hypothyroidism detection. Four of the five kits were found useful for clinical evaluation, the fifth less so.

Screening of Geriatric Patients for Thyroid Dysfunction with Thyrotropin-Releasing-Hormone Test, Sensitive Thyrotropin and Free Thyroxine Estimation

“…On the other hand, the predictive value of suppressed TSH and elevated FT4 with respect to clinical hyperthyroidism is insufficient, so that as a second step T4 and T3 estimation with better predictive value should be added to establish the diagnosis. Piketty, Talbot, Askienazy and Milhaud (1987), independently of geriatric patients, also stated that a suppressed TSH alone is not specific for hyperthyroidism but TSH can be used as the first screening test.…”

Sensitive thyrotrophin assays: excellent when properly used.

“…Another occasional problem is that in some assays spuriously detectable concentrations of thyrotrophin are recorded in hyperthyroidism because antibodies in the patient's serum bind to the reagent antibodies.4 On the other hand, thyrotrophin concentrations may be undetectable in euthyroid women in early pregnancy5 and in patients with severe endogenous depression. 6 The greatest source of concern, however, is in patients with non-thyroidal illness: in them biochemical assessment of thyroid function is notoriously difficult, particularly when kit methods for free thyroxine and triiodothyronine, using tracer analogues, are used.7 The newer thyrotrophin assays have greatly improved measurements in this group,' but diagnostic specificity is still impaired because of the pathophysiological suppression of thyrotrophin in severe illness (particularly in those patients receiving treatment with corticosteroids and dopamine9 10) and the rise in thyrotrophin occasionally seen during recovery. "…”