A case of biochemical assay discrepancy: Interference with measurement of thyroid-stimulating hormone due to rheumatoid factor

propylthiouracyl. In the case reported, an early diagnosis could not have been made because the patient had no clinical signs and was being treated with methamizole, with the resultant potential implications. It should be noted that the prevalence of liver parameter changes with the other two thioamides is so low that in a case such as the one reported, if the liver profile is not tested, this side effect may be overlooked. On the other hand, this has little practical relevance because the proportion of cases with asymptomatic elevations showing a poor course is probably very low. Because of the frequency and severity of the consequences, the measurement of liver function parameters, either at regular intervals or for any clinical suspicion, is clearly needed in subjects treated with propylthiouracyl. The question is whether routine liver profiles in patients treated with methimazole or carbimazole would be costeffective since, as already noted, liver iatrogeny with such drugs is uncommon and results in cholestatic patterns which may be diagnosed clinically. The prevalence of liver abnormalities with these drugs is very likely to be higher, but they possibly run an indolent course and resolve spontaneously.