Objective-Pregnancy is a time of rapidly changing demands on the thyroid axis, and knowledge of thyroid hormone levels, especially during the first trimester, is important for ensuring maternal and fetal health. The thyroid hormone assays currently in use become more inaccurate at extremes of binding protein concentrations and when heterophilic antibodies are present. Pregnancy is characterized by both these conditions, making accurate determination of free thyroid hormone levels by conventional direct analog immunoassay methods difficult. The objective of this study was to characterize the performance of a novel tandem mass spectrometric assay for free thyroxine during the physiologic conditions of pregnancy.Design-Healthy women without a history of thyroid abnormalities were recruited from the obstetrics and gynecology and endocrinology clinics of a university medical center and their thyroid status was monitored. Free thyroxine levels were assessed by both immunoassay and tandem mass spectrometry during the course of their pregnancy. Serum thyrotropin levels were also measured. The distributions of free thyroid concentrations obtained by the two assays were compared.Main outcome-The tandem mass spectrometry and immunoassay values did not correlate well with each other. However, tandem mass spectrometry values correlated well with the current gold standard equilibrium dialysis values. Moreover, the good agreement between equilibrium dialysis and tandem mass spectrometry was maintained across all weeks of gestation.Conclusions-We conclude that tandem mass spectrometry has a superior performance to immunoassay for the measurement of free thyroxine during pregnancy. Furthermore, it is ideally suited to generating trimester-specific reference intervals for free thyroxine levels. Future studies will determine if it is a better assay to use in most clinical circumstances.

BACKGROUND Accurate measurement of free thyroid hormones is important for managing thyroid disorders. Ultrafiltration liquid chromatography tandem mass spectrometry (LC-MS/MS) can reliably measure the concentrations of small molecules, including thyroid hormones. Our study was designed to compare free thyroid hormone measurements performed with immunoassay and LC-MS/MS. METHODS We studied the performance of LC-MS/MS in 4 different populations comprising pediatric patients, euthyroid adults, and healthy nonpregnant and pregnant women. The samples obtained from each population numbered 38, 200, 28, and 128, respectively. Free thyroxine, free triiodothyronine, and thyroid-stimulating hormone (TSH) concentrations were documented. RESULTS LC-MS/MS measurement of free thyroid hormones provided better correlation with log-transformed serum TSH in each population and also the populations combined. The correlations between free thyroxine measured by LC-MS/MS and log TSH in the pediatric outpatients and healthy adults were −0.90 and −0.77, respectively. The correlations for immunoassay were −0.82 and −0.48. The correlations between free triiodothyronine measured by LC-MS/MS and TSH for both pediatric and healthy adult populations were −0.72 and −0.68, respectively. CONCLUSIONS Free thyroid hormone concentrations measured by LC-MS/MS correlate to a greater degree with log TSH values compared to concentrations measured by immunoassay. This correlation was maintained across the patient populations we studied and may reflect the accuracy and specificity of LC-MS/MS. The superior ability of LC-MS/MS to enable documentation of the well-known thyroid hormone–TSH relationship supports the use of this measurement technique in a variety of clinical situations.

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In ReplyIn our earlier studies ultrafiltration (UF) 1 was performed at 25°C and not at room temperature or 37°C (1, 2 ). This was done to provide the endocrinologists at Children's National Medical Center with results that were essentially identical to those obtained by the then gold standard equilibrium dialysis immunoassay (EDIA) method marketed by Nichols. Comparison between these 2 approaches, EDIA and UF tandem mass spectrometry (UFMSMS) yielded the regression equation: EDIA ϭ 0.971 (UFMSMS) ϩ 0.041, r ϭ 0.954 (1 ), and we observed close agreement between the 2 methods (1, 2 ).The UF temperature used in the recently reported studies (3, 4 ) was also 25°C. Clearly the fact that this UFMSMS method yielded results that correlated well with both the gold standard EDIA method (performed at 37°C) and with log thyroid-stimulating hormone is a measure of the veracity and clinical utility of the UFMSMS method. Note that the direct analog IA correlation with EDIA/UFMSMS and log thyroid-stimulating hormone was poor (1-4 ). In the study population of pregnant women a correlation of r ϭ 0.898 was also observed between the EDIA gold standard method and UFMSMS at 25°C (3 ).Our initial studies (1, 2 ) reflecting the effect of UF temperature on free thyroxine (FT 4 ) and free triiodothyronine (FT 3 ) measured by UFMSMS have been extended and were recently published (5 ). The regression equations obtained for FT 4 respectively, and were tightly correlated (5 ). The Nichols kit is no longer available, and some commercial laboratories have developed EDMSMS methods that are performed at 37°C and use different membranes from those employed in the Nichols approach. These methods provide results approximately 1.5 times higher than the original Nichols gold standard method.At Georgetown University (Washington, DC) and NMS Laboratories (Willow Grove, PA) we have recently changed the temperature of the UF step to 37°C to both perform UF at the physiologically relevant temperature and harmonize our results to those from other commercial laboratories. This move from 25°C to 37°C increases the results and reference intervals by a factor of approximately 1.5 (5 ) but in no way affects data interpretation. Finally, it is noteworthy that the Endocrine faculty at Children's National Medical Center are happy with the FT 4 /FT 3 results they have routinely been getting over the past 3 years (Ͼ30 000 samples), measurements obtained with UFMSMS in which the UF is performed at 25°C. The Children's National Medical Center has refused to allow us to change the UF temperature to 37°C! The direct analog IA methods previously used at Children's National Medical Center have been discontinued because of their lack of credibility, and only UFMSMS FT 4 and FT 3 results are available.In summary, our laboratory has performed extensive comparisons between UFMSMS, performed at both 25°C and 37°C, and EDIA and EDMSMS. In all studies the results were in close agreement. In contrast to ED, UF is a rapid procedure and the precision afforded by UF is also far s...