Interference in thyroid-stimulating hormone determination

P>Background Thyroid-stimulating hormone (TSH) measurement plays a major role in the diagnosis of thyroid disorders. Despite the good quality of immunochemical tests measuring TSH levels, the presence of interfering substances can sometimes alter the TSH results. Design We reported the case of a 79-year-old man affected by primary autoimmune hypothyroidism hospitalized for pneumonia. A TSH value > 100 mIU L-1 (reference: 0.44 mIU L-1) was found at admission. No signs and symptoms of hypothyroidism were found upon clinical examination and serum concentration of the free thyroxine (FT4) was normal. Results Serum treatment in heterophile antibody blocking tubes did not change the TSH result in our assay, while normal levels were found in a different immunoassay method. An abnormal pattern was found in protein electrophoresis at admission, with IgG / j and IgM / k monoclonal bands proved in immunofixation. Interestingly, the disappearance of monoclonal bands was paralleled with a normalization of the TSH value. Conclusions We suggest in this study that the TSH determination might be influenced by the presence of transient paraproteins.