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Abstract

We thank Drs Midgley and Christofides for their insightful comments regarding free thyroxine (FT4) immunoassays. We would like to reiterate that we also believe it is important to establish pregnancy-specific reference ranges for FT4 immunoassays. However, such ranges may need to be specific to the population that is being studied, and this is not typically practical for clinical laboratories. Rather than debate the law of mass action and the validity of the free thyroxine index (FT4I), we want to emphasize that the purpose of the study was to evaluate the clinical utility of FT4 immunoassays when thyroid-stimulating hormone is not used as the primary test to assess thyroid function (ie, during anti–thyroid drug treatment for hyperthyroidism). This study was prompted in large part by the high number of referrals from obstetricians who asked us to evaluate patients with isolated hypothyroxinemia based on FT4 immunoassay results. We have observed, despite the controversy of whether thyroid function screening is necessary in pregnancy, that obstetricians routinely obtain complete thyroid function panels in their patients. We believe this practice is driven in part by the pressure exerted by the popular media that freely report associations of poor fetal outcome with abnormal thyroid function tests. Although the specifics regarding the superiority of 1 method vs another are left to debate, we believe our data reinforce the fact that the performance of these FT4 immunoassays is altered with pregnancy to a method-related degree. Until pregnancy-specific ranges are developed, history demonstrates that a normal FT4I or total T4 (with the use of an adjusted reference range for pregnancy) can be used to exclude the presence of hypothyroxinemia. We thank Drs Midgley and Christofides for their insightful comments regarding free thyroxine (FT4) immunoassays. We would like to reiterate that we also believe it is important to establish pregnancy-specific reference ranges for FT4 immunoassays. However, such ranges may need to be specific to the population that is being studied, and this is not typically practical for clinical laboratories. Rather than debate the law of mass action and the validity of the free thyroxine index (FT4I), we want to emphasize that the purpose of the study was to evaluate the clinical utility of FT4 immunoassays when thyroid-stimulating hormone is not used as the primary test to assess thyroid function (ie, during anti–thyroid drug treatment for hyperthyroidism). This study was prompted in large part by the high number of referrals from obstetricians who asked us to evaluate patients with isolated hypothyroxinemia based on FT4 immunoassay results. We have observed, despite the controversy of whether thyroid function screening is necessary in pregnancy, that obstetricians routinely obtain complete thyroid function panels in their patients. We believe this practice is driven in part by the pressure exerted by the popular media that freely report associations of poor fetal outcome with abnormal thyroid function tests. Although the specifics regarding the superiority of 1 method vs another are left to debate, we believe our data reinforce the fact that the performance of these FT4 immunoassays is altered with pregnancy to a method-related degree. Until pregnancy-specific ranges are developed, history demonstrates that a normal FT4I or total T4 (with the use of an adjusted reference range for pregnancy) can be used to exclude the presence of hypothyroxinemia. Free thyroxine assays: no going back!American Journal of Obstetrics & GynecologyVol. 202Issue 2PreviewLee et al1 have resurrected the free thyroxine index (FT4I) against modern analog-type (labeled analog/labeled antibody) free thyroxine (FT4) assays for diagnosis in pregnancy. This harkens back 40 years to a more innocent era, when shortcomings in thyroid hormone uptake methods were scarcely understood. All authentic FT4 assays must rigorously obey the law of mass action when measuring free hormone. Full-Text PDF