Endocrine Abstracts

A 67-year-old woman was diagnosed by her GP with hypothyroidism and referred to our department. During the first 2 years after diagnosis, the TSH remained elevated (16–19 mlU/l) with the free T4 ranging 11.1–31.9 pmol/l despite her insistence that compliance with therapy was good and on incremental doses of thyroxine. When seen in the nurse-led thyroid clinic for the first time, concerns were raised to the consultant endocrinologist. Interference with the TSH assay with heterophilic antibodies was suspected and reanalysis using the immunometric (sandwich) assay technique in the Roche modular system was performed. This had demonstrated that the TSH was undetectable with a high thyroxine level suggesting that heterophilic antibodies were indeed causing interference in the initial assay which although had used the same immunometric assay technique, had used a different analytical system, the Abbott Architect TSH method.

Heterophilic antibodies are circulating antibodies to animal immunoglobulins and can be of high or low affinity. Their origin is uncertain but they are present in 30–40% of the population, and present in higher proportions in populations that work with animals. Interference by human anti-mouse antibody (IgG) has been widely reported with TSH assays. Most modern immunoassay contain extensive ‘blocking’ reagents to prevent this phenomena but occasionally may still cause problems. For example, in over 200 000 immunoassays performed each year, possibly one patient will have the problem. The same person may have more than one test affected because of the design of the assays. In practice, when a result does not fit the clinical situation, one should consider the presence of heterophilic antibodies. This also raises important issues that we need to be aware of when monitoring and titrating thyroxine doses.