The application of an immunoradiometric assay of plasma thyrotropin (TSH-IRMA) in molar pregnancy

Plasma thyrotropin was measured by immunoradiometric assay (TSH-IRMA, "Sucrosep", Boots Celltech) in three patients with molar pregnancy. Two patients were hyperthyroid (increased plasma concentrations of FT4, FT4 index, and FT3 index), one patient was euthyroid (normal free thyroid hormone concentrations). TSH-IRMA values were below the detection limit (less than 0.1 mU/L) in the two hyperthyroid patients; the euthyroid patient had a TSH-IRMA value of 0.8 mU/L, within the reference range of normal subjects (0.5-3.3 mU/L). In vitro experiments demonstrated interference of the grossly elevated hCG levels in the TSH-IRMA, resulting in lower values. The observed interference was independent of ambient TSH concentrations, but related to hCG levels. We conclude that despite interference of hCG in TSH-IRMA the clinical usefulness of TSH-IRMA as a first-line test of thyroid function appears to be maintained in molar pregnancy.