Intermethod Discordant Free Thyroxine Measurements in Bone Marrow-transplanted Patients

Thyrotropin (TSH) has now been generally accepted as the first-line marker when screening for thyroid diseases. However, free thyroxine (FT4) measurements remain useful to confirm the diagnosis of thyroid diseases and to evaluate the thyroidal status when TSH measurements appear to disagree with the clinical picture, as may be the case in patients with nonthyroidal illnesses (1).Because it represents only a minute fraction of the total serum T4, measuring FT4, i.e., T4 not bound to binding proteins, remains difficult. This is especially true for sera from patients with nonthyroidal illnesses that have an altered binding capacity (2)(3). A variety of in vitro methods have been developed to estimate FT4. The low-molecular weight radioactive-labeled analog method was much criticized (2) and was soon completely withdrawn from the market. The most widely used methods are now the two-step assay and the one-step labeled-antibody assay. The aim of our study was to compare FT4 concentrations in sera from patients with nonthyroidal illnesses, measured with three recent nonisotopic automated techniques and with three manual RIAs: two immunoextraction assays (one two-step and one one-step assay), and a direct equilibrium dialysis (ED) assay considered as the reference method (2).The patient panel consisted of 20 hospitalized subjects on the seventh day following a bone marrow transplant, an example of severe acute illness. The bone marrow transplant was indicated for several hemopathies (n = 12) or solid cancers (n = 8). All patients were euthyroid before the treatment, and none had been known to have a patent thyroid dysfunction in the past. The preparation regimen before the transplant consisted of chemotherapy alone (n = 11) or chemotherapy associated with total-body radiotherapy (n = 9). After the transplant (allogenic in 9 cases and autogenic …