Clinical evaluation of nine free thyroxine assays: persistent problems in particular populations

Recent advances in liquid chromatography-tandem mass spectrometry for the detection of thyroid hormones and thyroglobulin in clinical samples: A review

Thyroid hormones (THs), including triiodothyronine (T3), thyroxine (T4), and their metabolites, are essential for regulating development, growth, and energy metabolism. Thyroglobulin (Tg) produced by thyroid follicular cells acts as an essential substrate for TH synthesis. The combination of THs with Tg is a widely used serological laboratory test for thyroid function assessment. Early detection and timely intervention are significant for preventing and managing thyroid disease. In recent years, liquid chromatography-tandem mass spectrometry (LC-MS/MS) has emerged as a powerful tool for the precise detection of small molecular analytes and steroid hormones in clinical practice as a result of its high sensitivity and specificity. While LC-MS/MS has been increasingly used for detecting THs and Tg recently, its application in clinical practice is still in its early stages. Recent advances in the assessment of thyroid metabolism using LC-MS/MS in clinical samples published during 2004-2023 were reviewed, with a special focus on the use of this technique for quantifying molecules involved in thyroid diseases.

Lower accuracy of testosterone, cortisol, and free T4 measurements using automated immunoassays in people undergoing hemodialysis

OBJECTIVES

Hormone measurements using automated immunoassays (IAs) can be affected by the sample matrix. Liquid chromatography tandem-mass spectrometry (LC-MS/MS) is less affected by these matrix effects. In clinical laboratories, testosterone, cortisol and, free thyroxine (FT4) are often measured using IAs. Renal failure alters serum composition in blood samples from people undergoing hemodialysis (HDp) and have, therefore, a complex serum constitution compared to healthy controls (HC). The goal of this study was to investigate the accuracy of testosterone, cortisol, and FT4 measurements in samples of HDp and to get more insight in the interfering factors.

METHODS

Thirty serum samples from HDp and HC were collected to measure testosterone, cortisol, and FT4 using a well standardized isotope dilution (ID)-LC-MS/MS method and 5 commercially available automated IAs (Alinity, Atellica, Cobas, Lumipulse, UniCel DXI). Method comparisons between LC-MS/MS and IAs were performed using both HDp and HC samples.

RESULTS

Average bias from the LC-MS/MS was for testosterone, cortisol, and FT4 immunoassays respectively up to 92, 7-47 and 16-27% more in HDp than in HC samples and was IA dependent. FT4 IA results were falsely decreased in HDp samples, whereas cortisol and testosterone concentrations in females were predominantly falsely increased. Correlation coefficients between LC-MS/MS and IA results were lower in HDp compared to HC samples.

CONCLUSIONS

Several IAs for testosterone (in women), cortisol, and FT4 are less reliable in the altered serum matrix of samples of HDp than in HC. Medical and laboratory specialists should be aware of these pitfalls in this specific population.

Hypothyroidism: The difficulty in attributing symptoms to their underlying cause

Common symptoms of overt hypothyroidism are non-specific and include fatigue, lethargy, and dry skin. Although the diagnosis is considered to be straightforward, no single symptom can be used to identify patients with overt hypothyroidism, while many patients with subclinical hypothyroidism are asymptomatic. A large population-based study on the spectrum of symptoms in subclinical hypothyroidism showed similar rates of thyroid disease-related symptoms compared with euthyroid subjects, while the TSH concentration had no impact on symptom score. Together, these findings make it challenging to attribute symptoms to their underlying cause. This is also true in the case of unexplained persistent symptoms in levothyroxine-treated patients. Although generally considered a life-long replacement therapy, successful thyroid hormone discontinuation resulting in euthyroidism has been reported in approximately one third of patients. Thus, we overtreat patients with (subclinical) hypothyroidism, highlighting the importance of reliable diagnostic criteria. The diagnostic process, including the implementation of robust TSH and FT4 reference intervals, is especially challenging in specific situations including aging, pregnancy, non-thyroidal illness, and central hypothyroidism. There is a clear need for improved adherence to current guidelines from scientific societies and for willingness to manage symptoms without a clear pathological correlate, especially in the case of mild TSH elevations. This review will highlight recent literature on this topic and offers some practice points.

Mass spectrometry in the diagnosis of thyroid disease and in the study of thyroid hormone metabolism

The importance of thyroid hormones in the regulation of development, growth, and energy metabolism is well known. Over the last decades, mass spectrometry has been extensively used to investigate thyroid hormone metabolism and to discover and characterize new molecules involved in thyroid hormones production, such as thyrotropin-releasing hormone. In the earlier period, the quantification methods, usually based on gas chromatography-mass spectrometry, were complicated and time consuming. They were mainly focused on basic research, and were not suitable for clinical diagnostics on a routine basis. The development of the modern mass spectrometers, mainly coupled to liquid chromatography, enabled simpler sample preparation procedures, and the accurate quantification of thyroid hormones, of their precursors, and of their metabolites in biological fluids, tissues, and cells became feasible. Nowadays, molecules of physiological and pathological interest can be assayed also for diagnostic purposes on a routine basis, and mass spectrometry is slowly entering the clinical laboratory. This review takes stock of the advancements in the field of thyroid metabolism that were carried out with mass spectrometry, with special focus on the use of this technique for the quantification of molecules involved in thyroid diseases.

Reference Intervals of Thyroid Function Tests Assessed by Immunoassay and Mass Spectrometry in Healthy Pregnant Women Living in Catalonia

BACKGROUND

Recent guidelines recommend establishing a local reference interval (RI) for thyroid function. We aimed to establish trimester-specific RIs for thyrotropin (TSH) and free thyroxine (FT4) in a cohort of healthy pregnant women in Catalonia (Spain).

METHODS

A prospective observational study was conducted with 332 healthy pregnant women, from the first trimester (1T) to delivery. TSH was measured using an Architect^® immunoassay (Abbott) and FT4 by two immunoassays, Architect^® (Abbott) and Cobas^® (Roche), in the three trimesters. FT4 was also measured by liquid chromatography mass spectrometry (LC/MS/MS) in the 1T.

RESULTS

TSH (µUI/mL) increased throughout pregnancy (1T: 0.03-3.78; 2T: 0.51-3.53; 3T: 0.50-4.32; p < 0.0001) and FT4 (pmol/L) progressively decreased (Architect^® 1T: 10.42-15.96; 2T: 8.37-12.74; 3T: 8.24-12.49; p < 0.0001; and Cobas^®: 1T: 11.46-19.05; 2T: 9.65-14.67; 3T: 8.88-14.54; p < 0.0067). The FT4 RI during 1T determined LC/MS/MS was 8.75-18.27. Despite the 1T FT4 results measured by LC/MS/MS and with the two immunoassays being significantly correlated, the results obtained by the three methods were found to be non-interchangeable.

CONCLUSIONS

We established trimester-specific RIs for TSH and for FT4 with immunoassays in our population. We also validated the 1T FT4 using LC/MS/MS to confirm the results of FT4 lower than the 2.5th percentile or higher than the 97.5th percentile.

Thyroid panel reference intervals in healthy children and adolescents: A Spanish cohort

OBJECTIVES

In children, thyroid hormones are essential for correct physical and neurological development. The recommended process for defining reference intervals (RIs) is the direct approach; however, indirect methods are an effective alternative. This study aimed to explore age- and sex-dependent relationships between serum concentrations of thyroid hormones in a large population-based cohort of healthy Spanish Caucasian children and calculate RIs.

MATERIAL AND METHODS

Results of serum thyroid-stimulating hormone (TSH) and free thyroxine (fT4) were collected from laboratory data of N (TSH = 23201; fT4 = 20728) patients aged 1 month - 15 years. These results were validated with a prospective study. Analyses of serum concentrations of TSH and fT4 were performed on ARCHITECT i2000 (Abbott Diagnostics, US). Percentiles (2.5th to 97.5th) were determined for each variable and taken as the RI.

RESULTS

No difference was found between serum TSH concentrations in male and female children of all age groups. A difference between serum fT4 concentrations in males and females and an age-dependent correlation for both sexes were found.

CONCLUSION

There is very little consensus on RIs in children. Our data confirm it is possible to use data mining techniques to calculate reliable and clinically useful RIs.

Trimester specific reference ranges for serum TSH and Free T4 among United Arab Emirates pregnant women

BACKGROUND

The American Thyroid Association (ATA) recommended the establishment of population specific reference ranges for thyroid hormones during pregnancy. Initial studies conducted in the United Arab Emirates (UAE) in 2003 and 2004 on pregnant women published a considerably higher upper limit for thyroid stimulating hormone (TSH) than that proposed by ATA. The UAE was classified as a country with mild iodine deficiency at the time of this initial study. After the implementation of aggressive strategies to address iodine deficiency over the last decade, the UAE was recently declared as iodine sufficient. The current study re-evaluates the reference intervals for thyroid hormones for pregnant women in the UAE after the declaration of iodine sufficiency status.

METHODS

TSH and free thyroxin (FT4) from 414 UAE national pregnant females were analyzed to determine trimester specific reference ranges.

RESULTS

The upper limits of the TSH reference ranges were found to be significantly lower than previously reported, but still higher than those recommended by ATA in 2011.FT4 reference ranges were found to be slightly lower than previously reported.

CONCLUSION

TSH trimester specific reference ranges in UAE national pregnant women are higher than those recommended by ATA in 2011 but in keeping with the latest guidelines published in 2017. This should be considered while interpreting thyroid function tests in this population. Further studies including urinary iodine measurement, body mass index and larger numbers per partition in this population are recommended.

Early pregnancy thyroid hormone reference ranges in Chilean women: the influence of body mass index

OBJECTIVE

Thyroid dysfunction and obesity during pregnancy have been associated with negative neonatal and obstetric outcomes. Thyroid hormone reference ranges have not been established for the pregnant Hispanic population. This study defines thyroid hormone reference ranges during early pregnancy in Chilean women and evaluates associations of body mass index (BMI) with thyroid function.

DESIGN, PATIENTS, MEASUREMENTS

This is a prospective observational study of 720 healthy Chilean women attending their first prenatal consultation at an outpatient clinic. Thyroid function [TSH, Free T4, Total T4 and antithyroid peroxidase antibodies (TPOAb)] and BMI were assessed at 8·8 ± 2·4 weeks of gestational age.

RESULTS

Median, 2·5th percentile (p2·5), and 97·5th percentile (p97·5) TSH values were higher, while median, p2·5, and p97·5 free T4 values were lower in obese patients compared with normal weight patients. Obesity was associated with a median TSH 16% higher (P = 0·035) and a median free T4 6·5% lower (P < 0·01) than values from patients with normal weight. BMI had a small, but statistically significant effect on TSH (P = 0·04) and free T4 (P < 0·01) when adjusted by maternal age, TPO antibodies, parity, sex of the newborn, gestational age and smoking. In all TPOAb (-) patients, median (p2·5-p.97·5) TSH was 1·96 mIU/l (0·11-5·96 mIU/l) and median (p2·5-p.97·5) free T4 was 14·54 pmol/l (11·1 - 19·02 pmol/l). Applying these reference limits, we found a prevalence of overt and subclinical hypothyroidism of 0·9% and 3·05% respectively.

CONCLUSIONS

TSH distributes at higher values and free T4 at lower values in obese pregnant women compared to normal weight pregnant women. Thyroid hormone reference ranges derived from Chilean patients with negative TPOAb are different from the fixed internationally proposed reference ranges and may be used in the Hispanic population.

DIAGNOSIS OF ENDOCRINE DISEASE: How reliable are free thyroid and total T3 hormone assays?

Hypothyroidism is a very common disorder worldwide, for which the usual treatment is monotherapy with levothyroxine (L-T₄). However, a number of patients treated with L-T₄ continue to report symptoms of hypothyroidism despite seemingly normal levels of thyroid-stimulating hormone (TSH), free-T₃ (FT₃) and free-T₄ (FT₄) measured by immunoassay. This review summarizes the limitations of the immunoassays commonly used to measure thyroid hormone levels and emphasizes the advantages of the role of liquid chromatography-tandem mass spectrometry (LC-MS/MS). Immunoassays for free thyroid hormone are affected by alterations in serum binding proteins that occur in many physiological and disease states. Multiple studies show falsely normal values for T₃, FT₃ and FT₄ by immunoassay that are below the reference interval when measured by (ultrafiltration) LC-MS/MS, a reference method. We suggest evaluation of thyroid hormone levels by ultrafiltration LC-MS/MS for patients who continue to experience hypothyroid symptoms on LT-4. This may help identify the approximately 20% subset of patients who would benefit from addition of T₃ to their treatment regimen (combination therapy).

Development of Standard Reference Materials to support assessment of iodine status for nutritional and public health purposes

The use of urinary iodine as an indicator of iodine status relies in part on the accuracy of the analytical measurement of iodine in urine. Likewise, the use of dietary iodine intake as an indicator of iodine status relies in part on the accuracy of the analytical measurement of iodine in dietary sources, including foods and dietary supplements. Similarly, the use of specific serum biomarkers of thyroid function to screen for both iodine deficiency and iodine excess relies in part on the accuracy of the analytical measurement of those biomarkers. The National Institute of Standards and Technology has been working with the NIH Office of Dietary Supplements for several years to develop higher-order reference measurement procedures and Standard Reference Materials to support the validation of new routine analytical methods for iodine in foods and dietary supplements, for urinary iodine, and for several serum biomarkers of thyroid function including thyroid-stimulating hormone, thyroglobulin, total and free thyroxine, and total and free triiodothyronine. These materials and methods have the potential to improve the assessment of iodine status and thyroid function in observational studies and clinical trials, thereby promoting public health efforts related to iodine nutrition.

Pediatric Reference Intervals for Free Thyroxine and Free Triiodothyronine by Equilibrium Dialysis-Liquid Chromatography-Tandem Mass Spectrometry

OBJECTIVE

Thyroid hormone concentrations fluctuate during growth and development. To accurately diagnose thyroid disease in pediatric patients, reference intervals (RIs) should be established with appropriate age groups from an adequate number of healthy subjects using the most exact methods possible. Obtaining statistically useful numbers of healthy patients is particularly challenging for pediatric populations. The objective of this study was to determine non-parametric RIs for free thyroxine (fT4) and free triiodothyronine (fT3) using equilibrium dialysis-high performance liquid chromatography-tandem mass spectrometry with over 2200 healthy children 6 months-17 years of age.

METHODS

Subjects were negative for both thyroglobulin and thyroid peroxidase autoantibodies and had normal thyrotropin concentrations. The study included 2213 children (1129 boys and 1084 girls), with at least 120 subjects (average of 125) from each year of life, except for the 6 month to 1 year age group (n=96).

RESULTS

Non-parametric RIs (95th percentile) for fT4 were: 18.0-34.7 pmol/L (boys and girls, 6 months-6 years) and 14.2-25.7 pmol/L (boys and girls, 7-17 years). RIs for fT3 were: 5.8-13.1 pmol/L (girls, 6 months-6 years); 5.7-11.8 pmol/L (boys, 6 months-6 years); 5.7-10.0 pmol/L (boys and girls, 7-12 years); 4.5-8.6 pmol/L (girls, 13-17 years); and 5.2-9.4 pmol/L (boys, 13-17 years).

CONCLUSION

Numerous significant differences were observed between pediatric age groups and previously established adult ranges. This emphasizes the need for well-characterized RIs for thyroid hormones in the pediatric population.

Thyroid Function in Pregnancy: What Is Normal?

BACKGROUND

Gestational thyroid dysfunction is common and associated with maternal and child morbidity and mortality. During pregnancy, profound changes in thyroid physiology occur, resulting in different thyroid-stimulating hormone (TSH) and free thyroxine (FT4) reference intervals compared to the nonpregnant state. Therefore, international guidelines recommend calculating trimester- and assay-specific reference intervals per center. If these reference intervals are unavailable, TSH reference intervals of 0.1–2.5 mU/L for the first trimester and 0.2–3.0 mU/L for the second trimester are recommended. In daily practice, most institutions do not calculate institution-specific reference intervals but rely on these fixed reference intervals for the diagnosis and treatment of thyroid disorders during pregnancy. However, the calculated reference intervals for several additional pregnancy cohorts have been published in the last few years and show substantial variation.

CONTENT

We provide a detailed overview of the available studies on thyroid function reference intervals during pregnancy, different factors that contribute to these reference intervals, and the maternal and child complications associated with only minor variations in thyroid function.

SUMMARY

There are large differences in thyroid function reference intervals between different populations of pregnant women. These differences can be explained by variations in assays as well as population-specific factors, such as ethnicity and body mass index. The importance of using correct reference intervals is underlined by the fact that even small subclinical variations in thyroid function have been associated with detrimental pregnancy outcomes, including low birth weight and pregnancy loss. It is therefore crucial that institutions do not rely on fixed universal cutoff concentrations, but calculate their own pregnancy-specific reference intervals.

Total and free thyroxine and triiodothyronine: measurement discrepancies, particularly in inpatients

OBJECTIVE

We compared the performance of tandem mass spectrometry versus immunoassay for measuring thyroid hormones in a diverse group of inpatients and outpatients.

METHODS

Thyroxine (T4), triiodothyronine (T3), free thyroxine (FT4), and free triiodothyronine (FT3) were measured by liquid chromatography tandem mass spectrometry and immunoassay in 100 patients and the two assays were compared.

RESULTS

T4 and T3 values measured by the two different assays correlated well with each other (r=0.91-0.95). However, the correlation was less good at the extremes (r=0.51-0.75). FT4 and FT3 concentrations measured by the two assays correlated less well with each other (r=0.75 and 0.50 respectively). The studied analytes had poor inverse correlation with the log-transformed TSH values (r=-0.22-0.51) in the population as a whole. The strongest correlations were seen in the groups of outpatients (r=-0.25-0.61). The weakest degree of correlation was noted in the inpatient group, with many correlations actually being positive.

CONCLUSION

The worst between-assay correlation was demonstrated at low and high hormone concentrations, in the very concentration ranges where accurate assay performance is typically most clinically important. Based on the lesser susceptibility of mass spectrometry to interferences from conditions such as binding protein abnormalities, we speculate that mass spectrometry better reflects the clinical situation. In this mixed population of inpatients and outpatients, we also note failure of assays to conform to the anticipated inverse linear relationship between thyroid hormones and log-transformed TSH.

Follow-up of newborns of mothers with Graves' disease

BACKGROUND

Overt neonatal Graves' disease is rare, but may be severe, even life threatening, with deleterious effects on neural development. The main objective of this study was to describe the course of thyrotropin (TSH) and free thyroxin (fT4) levels, as well as postnatal weight gain in relation to fT4 levels, in neonates born to women with Graves' disease without overt neonatal thyrotoxicosis. Such information is important to deduce the optimal schedule for evaluation.

METHODS

We conducted a retrospective chart review of neonates born to mothers with Graves' disease between January 2007 and December 2012. The records were reviewed for sex, gestational age, birth weight, maternal treatment during pregnancy, and maternal thyroid stimulating immunoglobulin (TSI) level. For each visit in the clinic, the data included growth parameters, presence of symptoms suspected for hyperthyroidism, blood test results (levels of TSH, fT4, and TSI), and treatment.

RESULTS

Ninety-six neonates were included in the study (49 males), with a total of 320 measurements of thyroid function tests (TSH and fT4). Four neonates (4%) had overt neonatal Graves' disease; one of them along with nine others were born preterm. In 77 (92.9%) of the remaining 83 neonates (the subclinical group), fT4 levels were above the 95th percentile on day 5. All had normal fT4 on day 15. A negative association was found between fT4 and weight gain during the first two weeks.

CONCLUSIONS

In this cohort, most neonates born to mothers with Graves' disease had a subclinical course with abnormal fT4 levels that peaked at day 5. After day 14, all measurements of fT4 returned to the normal range, although measurements of TSH remained suppressed for up to three months. Elevated fT4 was associated with poor weight gain.

Longitudinal assessment of thyroid function in pregnancy

Background

Trimester-specific reference intervals (RIs) for thyroid function tests are lacking for Beckman Dxl 800 analysers. We aimed to establish RIs for thyroid stimulating hormone (TSH), free thyroxine (fT4) and to track intraindividual changes in thyroid function throughout pregnancy.

Methods

One hundred and thirty healthy women without antithyroid peroxidase antibodies were followed longitudinally. Thyroid function was determined at trimester-1 (T1): 9–13 weeks; trimester-2 (T2): 22–26 weeks; trimester-3 (T3): 35–39 weeks and postpartum (PP): 8–12 weeks. A subgroup ( n = 47) was used to track intraindividual changes using PP as non-pregnant state (baseline).

Results

For trimesters 1–3, TSH (median (2.5th, 5th, 95th and 97.5th percentile)) was 0.77 (0.03, 0.05, 2.33, 3.05), 1.17 (0.42, 0.47, 2.71, 3.36) and 1.35 (0.34, 0.42, 2.65, 2.83) mIU/L, respectively. Free T4 (mean (95%CI)) was 10.7 (5.9–15.5), 8.1 (4.9–11.3), 7.8 (4.5–11.0) pmol/L, respectively. In T2 and T3, 36% and 41% of the fT4 values, respectively, fell below the non-pregnancy lower normal limit. In the subgroup assessed for longitudinal changes, of the women with baseline TSH ⩽ median, 71–75% remained at or below the corresponding median for trimesters 1–3. Of the women with baseline fT4 ⩽ median, 69–81% also remained at or below the corresponding median for trimesters 1–3. High correlation was observed at different trimesters and baseline for TSH (Spearman’s r: 0.593–0.846, P < 0.001) and for fT4 ( r: 0.480–0.739, P < 0.001).

Conclusions

Use of trimester-specific RIs would prevent misclassification of thyroid function during pregnancy. In the majority of women, TSH and fT4 tracked on the same side of the median distribution, from a non-pregnant baseline, throughout pregnancy.

The expanding role of tandem mass spectrometry in optimizing diagnosis and treatment of thyroid disease

This review discusses the state-of-the-art measurement of free and total thyroid hormones in clinical laboratories. We highlight some of the limitations of currently used immunoassays and critically discuss physical separation methods for the measurement of free thyroid hormone. Physical separation methods, such as equilibrium dialysis or ultrafiltration, followed by tandem mass spectrometry for the measurement of free thyroid hormones offer many advantages, which we feel, can deepen our understanding of thyroid hormone metabolism and improve patient diagnosis and care. Problems with direct analogue immunoassay methods for FT4/FT3 as well as immunoassay methods for total T3 at low T3 concentrations and during pregnancy are highlighted. Improved diagnosis and patient management can be achieved utilizing tandem mass spectrometry for these measurements.

Hypothyroxinemia and pregnancy

OBJECTIVE

To evaluate the peer-reviewed literature on iodine deficiency and hypothyroxinemia in pregnancy.

METHODS

We review published studies on isolated hypothyroxinemia in pregnancy, methodology of free thyroxine (T4) assays, impact of iodine deficiency on free T4 levels, and status of ongoing prospective randomized trials of isolated hypothyroxinemia during pregnancy.

RESULTS

Hypothyroxinemia during pregnancy is common. Studies have demonstrated the pivotal role exerted by maternal T4 on fetal brain development and the negative impact of hypothyroxinemia on neurobehavioral performance in offspring. Two intervention studies have demonstrated a positive effect on neurodevelopment in children of mothers promptly supplemented with iodine compared with the neurodevelopment in children of nonsupplemented mothers. Free T4 assays presently in clinical use have limitations. Preliminary results of the Controlled Antenatal Thyroid Study (CATS) are somewhat mixed, and the National Institutes of Health Maternal Fetal Medicine Thyrotropin Study (TSH Study) will be completed in 2015. Knowledge regarding the impact of isolated hypothyroxinemia has progressed, but major questions remain. An optimal diagnostic test for free T4 during pregnancy (accurate, inexpensive, and widely available) remains elusive. Trimester-specific normative data and normal ranges from different geographic regions do not exist.

CONCLUSIONS

Data published to date are insufficient to recommend levothyroxine therapy in pregnant women with isolated hypothyroxinemia. Adequate iodine intake should be recommended before conception and early in pregnancy.

Levothyroxine treatment in pregnancy: indications, efficacy, and therapeutic regimen

The prevalence of overt and subclinical hypothyroidism during pregnancy is estimated to be 0.3–0.5% and 2–3%, respectively. Thyroid autoantibodies are found in 5–18% of women in the childbearing age. The aim of this review is to underscore the clinical significance of these findings on the health of both the mother and her offspring. Methods of evaluation of thyroid function tests (TFTs) during pregnancy are described as are the threshold values for the diagnosis of overt and subclinical hypothyroidism or hypothyroxinemia. Anticipated differences in TFTs in iodine-sufficient and iodine-deficient areas are discussed and data are provided on potential complications of hypothyroidism/hypothyroxinemia and autoimmune thyroid disease during pregnancy and adverse effects for the offspring. The beneficial effects of levothyroxine therapy on pregnancy outcomes and offspring development are discussed with a proposed treatment regimen and follow up strategy.

Stability of thyroid hormones during continuous infusion

We investigated the stability of thyroid hormones during a mode of continuous drug infusion via polypropylene tubing using the same conditions that would be applied to treating patients in a hospital setting. The diluted thyroid hormones were prepared using aseptic technique, stored at 2-8°C (36-46°F) and tested within 24 h of preparation for stability and percent recovery from within plastic tubing. Experiments were done in duplicate with triplicate sets of readings for each assay point. Only T(4) prepared with 5% dextrose water (D5W) containing 1 mg/mL albumin remained constant, stable, predictable and accurate over time under various conditions. Other methods of preparation lost drug by adhering to the plastic containers and tubing by as much as 40% of starting concentration. T(3) recovery in the presence of 1 mg/mL of albumin was 107±2% (mean±standard error of the mean) of anticipated drug concentrations. We conclude from this series of experiments that to maintain an accurate and stable dosing of patients receiving intravenous thyroid hormones, 1 mg/mL of albumin must be added to the infusate to prevent lost on the plastic intravenous tubing.

Doubts and Concerns about Isolated Maternal Hypothyroxinemia

There is evidence that isolated maternal hypothyroxinemia may have detrimental effects on both mother and foetus. Nonetheless, this condition is still far from being universally accepted as a separate thyroid disease, and a standard definition of this state of mild thyroid underfunction is still lacking. We will review the biochemical criteria used to define isolated maternal hypothyroxinemia, together with current methodological issues related to FT4 assays. We will also discuss its epidemiological impact in both iodine-deficient and-sufficient areas, and the effectiveness of iodine prophylaxis on maternal thyroid function and neuropsychomotor development in offspring.

Inverse log-linear relationship between thyroid-stimulating hormone and free thyroxine measured by direct analog immunoassay and tandem mass spectrometry

BACKGROUND

Accurate measurement of free thyroxine (FT(4)) is important for diagnosing and managing thyroid disorders. Most laboratories measure FT(4) by direct analogue immunoassay methods. The validity of these methods have recently been questioned. The inverse log-linear relationship between FT(4) and thyroid-stimulating hormone (TSH) is well described and provides a physiological rationale on which to base an evaluation of FT(4) assays.

METHODS

The study included 109 participants for whom FT(4) measurement was requested by their clinician. Samples were selected for inclusion to reflect a wide spectrum of TSH and albumin results. FT(4) and TSH were measured by use of the Siemens Immulite immunoassay (IA). FT(4) was also measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) (MS-FT(4)).

RESULTS

The inverse log-linear correlation coefficient between TSH and FT(4) was significantly better (P < 0.0001) for MS-FT(4) (0.84, 95% CI, 0.77-0.88) than for IA-FT(4) (0.45, 95% CI, 0.29-0.59). IA-FT(4) showed a significant correlation with albumin (Spearman correlation coefficient 0.45, 95% CI, 0.29-0.5, P < 0.0001) and thyroxine-binding globulin (TBG) (Spearman correlation coefficient 0.23, 95% CI, 0.05-0.41, P = 0.02). In contrast, FT(4) measurement by LC-MS/MS did not show a significant correlation with albumin or TBG.

CONCLUSIONS

The inverse log-linear relationship between FT(4) and TSH was significantly better for FT(4) measured by LC-MS/MS than by IA. The MS-FT(4) method therefore provides FT(4) results that agree clinically with those obtained for TSH. Additionally, the significant correlation between IA-FT(4) with albumin and TBG suggests that this FT(4) method depends on binding protein concentrations and consequently does not accurately reflect FT(4).

Therapeutic drug monitoring during pregnancy and lactation: thyroid function assessment in pregnancy-challenges and solutions

The diagnosis and monitoring of thyroid disease necessitates the knowledge of thyroid pathophysiology and of the technical limitations of current thyroid-related biochemical tests. Thyroid disease diagnosis and monitoring are further complicated during pregnancy and lactation, due to pregnancy-related changes in thyroid hormone metabolism. Dramatic changes that occur in thyroxine and triiodothyronine ranges during pregnancy pose challenges for hypothyroid gravidas. Very early in pregnancy, levothyroxine replacement needs to be increased. Moreover, increases in thyroid hormone replacement need to be conducted individually and on a timely basis. For reasons that are still not entirely clear, although dependent in part on changes in thyroxine binding, free thyroxine (FT4) levels decrease as pregnancy progresses necessitating the use of trimester-specific reference intervals for appropriate replacement. Thyroxine binding protein levels vary by hormonal status, inheritance, and disease states and are higher in pregnancy; hence, FT4 assays became popular because they measure the unbound hormone. However, current FT4 immunoassays are estimate tests that do not reliably measure FT4 and are known to be sensitive to alterations in binding proteins and therefore are method-specific. The need to reliably identify hypothyroxinemic pregnant patients, especially in the first trimester, is of prime importance for early fetal brain development before the fetal thyroid functions. This article addresses 1) the current limitations of laboratory-free thyroxine immunoassay methodologies and especially during pregnancy; 2) trimester-specific reference intervals for thyroid function tests; and 3) the study of levothyroxine pharmacokinetics in pregnant and nonpregnant women.

Thyroid hormone testing by tandem mass spectrometry

In the euthyroid person, absolute free thyroxine concentrations remain constant and correlate with the tissue hormone level, its biologic effect and the metabolic status of the patient. However, most circulating thyroid hormone is bound to plasma proteins and only a minute amount is in the unbound free form. Studies have shown that current free thyroxine immunoassays are binding protein dependent. Novel high-performance liquid chromatography tandem mass spectrometry (LC/MS/MS) methods have successfully dealt with problems inherent in many immunoassays for thyroid hormones and afforded improved specificity and accuracy in thyroid hormone measurements. We emphasize problems with thyroid hormone testing employing immunoassays including direct and indirect thyroid hormone immunoassays, sample processing, methods of free hormone separation and review the emerging role of liquid chromatography-tandem mass spectrometry in thyroid hormone testing. The latest generation of tandem mass spectrometers has superior limits of quantification, permitting omission of previously employed derivatization steps. Liquid chromatography-tandem mass spectrometry affords the specificity, precision, and limits of quantification necessary for the reliable measurement of thyroid hormones, enhancing diagnostic capabilities, and affording the profiles of the iodothyronines and thyronamines. These methods are especially important in states of disease and during pregnancy when protein binding is a factor that interferes with other methods for thyroid hormone analysis.

Agreement of immunoassay and tandem mass spectrometry in the analysis of cortisol and free t4: interpretation and implications for clinicians

Objective. To quantify differences in results obtained by immunoassays (IAs) and tandem mass spectrometry (MSMS) for cortisol and free thyroxine (FT4). Design & Patients. Cortisol was measured over 60 minutes following a standard ACTH stimulation test (n = 80); FT4 was measured over time in two cohorts of pregnant (n = 57), and nonpregnant (n = 28) women. Measurements. Samples were analyzed with both IA and MSMS. Results. Results for cortisol by the two methods tended to agree, but agreement weakened over the 60-minute test and was worse for higher (more extreme) concentrations. The results for FT4 depended on the method. IA measurements tended to agree with MSMS measurements when values fell within “normal levels”, but agreement was not constant across trimester in pregnant women and was poorest for the extreme (low/high) concentrations. Correlations between MSMS measurements and the difference between MSMS and IA results were strong and positive (0.411 < r < 0.823; all P < .05). Conclusions. IA and MSMS provide different measures of cortisol and FT4 at extreme levels, where clinical decision making requires the greatest precision. Agreement between the methods is inconsistent over time, is nonlinear, and varies with the analyte and concentrations. IA-based measurements may lead to erroneous clinical decisions.

Complex relationship between free thyroxine and TSH in the regulation of thyroid function

OBJECTIVE

The present study re-evaluates the inverse log TSH-free thyroxine (fT(4)) relationship, which has generally been assumed to characterize the thyroid pituitary hypothalamic feedback regulation in thyroid function.

DESIGN AND METHODS

The correlation between fT(4) and TSH was analyzed in two data sets from differing time periods involving 3223 and 6605 patients referred for thyroid testing, representing the whole range of thyroid functions from hypothyroidism to hyperthyroidism.

RESULTS

We found that the data do not support a linear log TSH-fT(4) relationship; instead, the correlation's gradient varies with thyroid function. As a consequence, an alternate model, based on the error function, was introduced. When directly comparing the models by means of curve fitting, using F-test and Akaike criteria, the alternate model results in a significantly better fit. The model was verified in the independent second set of data. Subgroup analysis of untreated patients added further proof to the non-linear model.

CONCLUSIONS

We propose a refined non-linear model to describe the relationship between TSH and fT(4). It implies that TSH response to a deviating fT(4) value may not be log-linear, but may be disproportionally related to the extent of the deviation from an optimum set point. A better understanding of the complex nature of the TSH-fT(4) relationship may further the development of more precise clinical models and aid in better defining subclinical states of thyroid dysfunction. Also, it may encourage other biological interrelations to be reconsidered in the wake of advanced measurement techniques and more powerful computerized statistical procedures.

Pediatric reference intervals for free thyroxine and free triiodothyronine

BACKGROUND

The clinical value of free thyroxine (FT(4)) and free triiodothyronine (FT(3)) analysis depends on the reference intervals with which they are compared. We determined age- and sex-specific reference intervals for neonates, infants, and children 0-18 years of age for FT(4) and FT(3) using tandem mass spectrometry.

METHODS

Reference intervals were calculated for serum FT(4) (n = 1426) and FT(3) (n = 1107) obtained from healthy children between January 1, 2008, and June 30, 2008, from Children's National Medical Center and Georgetown University Medical Center Bioanalytical Core Laboratory, Washington, DC. Serum samples were analyzed using isotope dilution liquid chromatography tandem mass spectrometry (LC/MS/MS) with deuterium-labeled internal standards.

RESULTS

FT(4) reference intervals were very similar for males and females of all ages and ranged between 1.3 and 2.4 ng/dL for children 1 to 18 years old. FT(4) reference intervals for 1- to 12-month-old infants were 1.3-2.8 ng/dL. These 2.5 to 97.5 percentile intervals were much tighter than reference intervals obtained using immunoassay platforms 0.48-2.78 ng/dL for males and 0.85-2.09 ng/dL for females. Similarly, FT(3) intervals were consistent and similar for males and females and for all ages, ranging between 1.5 pg/mL and approximately 6.0 pg/mL for children 1 month of age to 18 years old.

CONCLUSIONS

This is the first study to provide pediatric reference intervals of FT(4) and FT(3) for children from birth to 18 years of age using LC/MS/MS. Analysis using LC/MS/MS provides more specific quantification of thyroid hormones. A comparison of the ultrafiltration tandem mass spectrometric method with equilibrium dialysis showed very good correlation.

Correlations of free thyroid hormones measured by tandem mass spectrometry and immunoassay with thyroid-stimulating hormone across 4 patient populations

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.

Reference intervals in evaluation of maternal thyroid function during the first trimester of pregnancy

BACKGROUND

Insufficient function of the thyroid gland can cause low concentrations of thyroid gland hormones which are necessary for the proper development of the foetus brain, especially during the beginning of pregnancy. We determined the reference interval for TSH in the first trimester of pregnancy and the decision point for anti-TPO antibodies' positivity.

METHODS

We tested 5520 women from the central part of the Czech Republic during the first trimester of pregnancy for serum TSH, anti-TPO and FT4 by chemiluminometric immunoanalysis on an ADVIA Centaur system (Siemens). The reference interval for TSH during the first trimester of pregnancy was determined using the log transformation, and then summarized as the geometrical mean (95% CI); following which, a suitable decision point for anti-TPO positivity was set.

RESULTS

The reference interval for TSH was determined to be 0.06-3.67 mU/l. The suppression of TSH was found in 2.93% of the women; a raised concentration of TSH had been found in 4.48% of the women. For anti-TPO, the cut-off was established at 143 kU/l; 11.2% of the pregnant women were found to be anti-TPO positive.

CONCLUSION

Determination of the reference interval for TSH during early pregnancy is one of the basic requirements when implementing a general examination of the thyroid gland at the beginning of pregnancy. The decision level for positivity of anti-TPO is more than double the manufacturer's reference interval.

Tandem mass spectrometry as a novel tool for elucidating pituitary-thyroid relationships

BACKGROUND

Our objective was to determine the performance of liquid chromatography-tandem mass spectrometry (LC-MS/MS) in documenting both group and individual relationships between thyroid hormone and thyroid-stimulating hormone (TSH) concentrations.

METHODS

This was a prospective analysis of 50 euthyroid patients undergoing thyroidectomy. Thyroxine (T(4)), triiodothyronine (T(3)), free T(4) (FT(4)), and TSH levels were documented on two occasions before thyroidectomy. After thyroidectomy, patients were treated with levothyroxine (LT(4)) to achieve either a normal or low serum TSH concentration. All laboratory evaluations were repeated twice while patients were taking LT(4). Thyroid hormone concentrations were documented by both immunoassay and LC-MS/MS, and their relationship with TSH was studied both in the entire group and in individual patients pre- and postthyroidectomy.

RESULTS

FT(4) and total T(3) correlated better with the log-transformed TSH when measured by LC-MS/MS. Postthyroidectomy the closest correlation was between log TSH and FT(4) (r = 0.86, p < 0.001). The next best correlation was between log TSH and total T(3) (r = 0.71, p < 0.001). When all data points were combined, the slope of the relationship between log TSH and total T(3) was relatively blunted compared with the log TSH-FT(4) slope (slope - 0.39 vs. - 1.38; p < 0.001), perhaps suggesting autoregulation of T(3) in response to the altered conditions postthyroidectomy.

CONCLUSION

LC-MS/MS is an excellent tool for documenting the known physiological phenomenon of a log-linear relationship between TSH and thyroid hormone concentrations. In a group of patients studied pre- and postthyroidectomy, both FT(4) and total T(3) measured by tandem mass spectrometry correlate well with TSH. However, T(3) correlates slightly less well and has a relatively blunted relationship with the log-transformed TSH. These paired data suggest that in LT(4)-replaced patients T(3) concentrations are held stable in the face of fluctuating T(4) concentrations.

Pediatric reference intervals for thyroid hormone levels from birth to adulthood: a retrospective study

BACKGROUND

Age- and sex-specific reference intervals are an important prerequisite for interpreting thyroid hormone measurements in children. However, only few studies have reported age- and sex-specific pediatric reference values for TSHbasal (TSH), free T3 (fT3), and free T4 (fT4) so far. Reference intervals are known to be method- and population-dependent. The aim of our study was to establish reference intervals for serum TSH, fT3, and fT4 from birth to 18 years and to assess sex differences.

METHODS

2,194 thyroid hormone tests obtained from a hospital-based pediatric population were included into our retrospective analysis. Individuals with diagnoses or medications likely to affect thyroid function were primarily excluded, as well as the diagnostic groups, if different from the purely healthy subgroup (n = 414). Age groups were ranging from 1 day to 1 month, 1 - 12 months, and 1 - 5, 6 - 10, 11 - 14, and 15 - 18 years, respectively. Levels of fT3, fT4 and TSH were measured on Advia(R) Centaur automated immunoassay system.

RESULTS

The final sample size for reference data creation was 1,209 for TSH, 1,395 for fT3, and 1,229 for fT4. Median and 2.5/10/25/75/90/97.5 percentiles were calculated for each age group. Males had greater mean fT3 concentrations than females (p < 0.001). No sex-differences were found for TSH and fT4 between age-matched serum samples. Median concentrations of fT3, fT4 and TSH were greatest during the first month of life, followed by a continuous decline with age.

CONCLUSION

Our results corroborate those of previous studies showing that thyroid hormone levels change markedly during childhood, and that adult reference intervals are not universally applicable to children. Moreover, differences of our reference intervals compared to previous studies were observed, likely caused by different antibody characteristics of various analytical methods, different populations or undefined geographic covariates, e.g. iodine and selenium status.

Simultaneous quantification of free triiodothyronine and free thyroxine by isotope dilution tandem mass spectrometry

OBJECTIVES

This study was designed to improve our previously developed tandem mass spectrometry (MS/MS) method for free thyroxine (FT4) by enhancing sensitivity and permitting simultaneous measurements of both free triidothyronine (FT3) and FT4 using a smaller plasma/serum sample.

DESIGN AND METHODS

An API-5,000 tandem mass spectrometer equipped with TurboIonSpray source and Shimadzu HPLC system was employed to perform the analysis using isotope dilution with deuterium labeled internal standard, T4-d(5). Four hundred microliters of human plasma/serum was filtered through a Centrifree YM-30 ultrafiltration device by centrifugation, and 450 microL of internal standard in methanol was then added to 150 microL of ultrafiltrate for deproteinization. After centrifugation, 500 microL of supernatant was diluted with 400 microL of distilled de-ionized water and a 650 microL aliquot was injected onto a C-18 column. After washing, the switching valve was activated and the analytes were eluted from the column with a water/methanol gradient into the MS/MS system. Quantification by multiple reaction-monitoring (MRM) analysis was performed in the negative mode.

RESULTS

The within-day and between-day coefficients of variation (CVs) were <or=9% for FT3 and <or=7% for FT4 at all concentrations tested. Accuracy ranged between 95% and 105%. The 2.5th-97.5th percentile for FT3 and FT4 was 0.09-0.4 ng/dL (1.4-6.2 pmol/L) and 0.8-2.1 ng/dL (10-26 pmol/L), respectively. The results correlated only moderately well with the immunoassays.

CONCLUSIONS

We describe an improved simple, accurate and fast isotope dilution tandem mass spectrometry method for the simultaneous determination of FT3 and FT4 in human serum/plasma samples.

Tandem mass spectrometry improves the accuracy of free thyroxine measurements during pregnancy

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.

Use of Frozen Sera for FT4 Standardization: Investigation by Equilibrium Dialysis Combined with Isotope Dilution-Mass Spectrometry and Immunoassay

Background: Serum-free thyroxine (FT4) testing is recommended for diagnosis or monitoring of thyroid dysfunction, particularly in cases of hormone binding abnormalities. However, the poor intermethod agreement among commercial FT4 assays suggests a need for standardization with a hierarchically higher measurement procedure. To that purpose, we applied equilibrium dialysis (ED) in combination with isotope dilution-liquid chromatography/tandem mass spectrometry (ID-LC-tandem MS).

Methods: After ED, we collected dialysate into tubes containing [13C6]-T4 for ID and [13C9]-T4 as carrier, purified the samples by solid-phase extraction, and analyzed them with LC/tandem MS. We evaluated the procedure’s analytical performance and tested its suitability for measurement of hypo-, eu-, and hyperthyroid serum FT4 concentrations. We conducted a pilot method comparison study with 3 commercial assays to investigate whether frozen sera could be used for the purpose of FT4 standardization.

Results: The within-run, between-run, and total CVs (inclusive ED) were 3.7%, 4.2%, and 5.6%, respectively (17.7 pmol/L; n = 20). The mean accuracy, estimated from recovery experiments with dialysate and dialysis buffer supplemented at 8.7, 18.7, and 33.5 pmol/L, and from analysis of certified sera gravimetrically diluted to 9.8, 19.2, and 34.8 pmol/L, was 98.0% to 102.8%. The procedure’s limit of detection and limit of quantification were 0.5 and 1.3 pmol/L, respectively. The method comparison demonstrated the suitability of the selected sera for standardization of FT4 assays and confirmed the lack of assay comparability.

Conclusions: We demonstrated that the described ED-ID-LC/tandem MS procedure and the selected type of sera qualify for standardization of FT4 measurements.

Thyrotrophin and free thyroxine trimester-specific reference intervals in a mixed ethnic pregnant population in the United Arab Emirates

BACKGROUND

Physiological alterations in the homeostatic control of thyroid hormones cause changes in thyroid function tests in pregnant women. A lack of method, trimester and population-specific reference intervals for free thyroxine (FT4) and thyrotrophin (TSH) makes interpretation of FT4 and TSH levels in pregnancy difficult. We established trimester-specific reference intervals for TSH and FT4 in a mixed ethnic population of pregnant women attending two antenatal clinics in the United Arab Emirates.

METHODS

TSH and FT4 result from 1140 women with uncomplicated singleton pregnancy were available. The 95% reference intervals were determined for TSH and FT4 for each trimester for Arab women from the United Arab Emirates and other Arab countries and Asian women.

RESULTS

Suppressed TSH levels in the first trimester recovered to non-pregnant levels in the third trimester. There was a significant difference in TSH levels between trimesters 1 and 2, and 2 and 3 (p<0.0005). There was no significant difference in the TSH levels between the various ethnic groups. Mean FT4 levels decreased with each progressive trimester in all groups. There were significant differences in FT4 levels between all three trimesters (p<0.005), especially between the first and second trimesters. FT4 differed significantly between UAE nationals and Asians in the first and second trimesters (p<0.005).

CONCLUSIONS

In general, the findings were in keeping with earlier reports. Use of trimester-specific reference intervals should help in the appropriate interpretation of thyroid hormone results in the mixed UAE population.

Thyroid hormone reference intervals in an ambulatory Arab population on the Abbott Architect i2000 immunoassay analyzer

BACKGROUND

Considerable differences in reference intervals for FT4 and TSH have been reported between countries. Method related differences in the distribution of free thyroxine (FT4) have also been reported. The aim of this study was to establish reference intervals for thyrotrophin (TSH) and FT4 in an ambulatory adult (16-75 y) Arab population attending a general practice clinic using the Abbott Architect i2000 immunoassay analyzer.

METHODS

TSH and FT4 results from 959 consecutive ambulatory Arab subjects were available. After excluding data sets from pregnant women, patients with known and newly diagnosed thyroid disease, individuals taking medication that may affect TSH and FT4 and individuals with acute illness, 742 data sets were available for analysis. A 2-way between-groups ANOVA was conducted to explore the impact of age and gender on TSH and FT4.

RESULTS

TSH showed a non-Gaussian distribution, FT4 showed a near normal distribution. There was no significant main effect on FT4 and TSH for age and gender. The interaction effect of age and gender also did not reach significance. The 95% reference intervals were: TSH 0.30-4.32 mU/l and FT4 9.8-18.6 pmol/l. The reference intervals for TSH and FT4 determined in this study differed from those reported from other countries using the same analytical platform and from the 99% reference intervals, provided by the manufacturer.

CONCLUSIONS

These differences in reference intervals in different populations may affect patient management. The data reported reemphasize that each laboratory should determine population and method-specific reference intervals.

Are detection and treatment of thyroid insufficiency in pregnancy feasible?

A workshop entitled, "The Impact of Maternal Thyroid Diseases on the Developing Fetus: Implications for Diagnosis, Treatment, and Screening," was held in Atlanta, Georgia, January 12-13, 2004. The workshop was sponsored jointly by The National Center on Birth Defects and Developmental Disabilities of The Centers for Disease Control and Prevention (CDC) and The American Thyroid Association. This paper reports on the individual session that examined the ability to detect and treat thyroid dysfunction during pregnancy. For this session, presented papers included: "Laboratory Reference Values in Pregnancy" and "Criteria for Diagnosis and Treatment of Hypothyroidism in Pregnancy." These presentations were formally discussed by invited respondents and by others in attendance. Salient points from this session about which there was agreement include the following: thyrotropin (TSH) can be used as marker for hypothyroidism in pregnancy, except when there is iodine deficiency usually evidenced by elevated serum thyroglobulin (Tg). We need more longitudinal studies of TSH during pregnancy in iodine-sufficient populations without evidence of autoimmune thyroid disease to develop trimester-specific TSH reference ranges. Current free thyroxine (FT4) estimate methods are sensitive to abnormal binding-protein states such as pregnancy. There is no absolute FT4 value that will define hypothyroxinemia across methods. Total thyroxine (TT4) changes in pregnancy are predictable and not method-specific. TT4 below 100 nmol/L (7.8 microg/dL) is a reasonable indicator of hypothyroxinemia in pregnancy. Women with known hypothyroidism and receiving levothyroxine (LT4) before pregnancy should plan to increase their dosage by 30% to 60% early in pregnancy. Women with autoimmune thyroid disease prior to pregnancy are at increased risk for thyroid insufficiency during pregnancy and postpartum thyroiditis and should be monitored with TSH during pregnancy.

Differences between measurements of T4 and T3 in pregnant and nonpregnant women using isotope dilution tandem mass spectrometry and immunoassays: are there clinical implications?

BACKGROUND

It has been established that triiodothyronine (T3) and thyroxine (T4) measurements by tandem mass spectrometry (MS/MS) are more specific and are significantly different from immunoassay (IA) measurements (all p< or =0.05) throughout pregnancy. In this study, we examined the clinical implications of these discrepancies.

METHODS

Kappa statistics were used to determine the degree to which IA and MS/MS agreed in their identification of out-of-reference interval observations of circulating T4 and T3 from 52 normal, iodine-sufficient women during each trimester of pregnancy.

RESULTS

After taking chance agreement into account, the two methods had poor agreement on classification of T3 values at the first (kappa=0.185) and second (kappa=0.183) trimesters, with extremely poor agreement for the third trimester and 1-year postpartum. Agreement on T4 was poor only for the third trimester (kappa=0.183). The two methods agreed on out-of-range values in only 0-25% of T3 cases and 25-66.7% of T4 cases.

CONCLUSIONS

The areas of disagreement suggest that women at risk (i.e., with analyte values outside of the 5-95% range) will not be detected using IA. Based on this cohort, our preliminary estimates are that 25-100% of such women would be missed if IA were used to assay the analyte.

Trimester-specific changes in maternal thyroid hormone, thyrotropin, and thyroglobulin concentrations during gestation: trends and associations across trimesters in iodine sufficiency

OBJECTIVES

To describe the interrelationships of thyroid functions based on trimester-specific concentrations in healthy, iodine-sufficient pregnant women across trimesters, and postpartum.

METHODS

Circulating total 3,5,3'- triidothyronine (T(3)) and thyroxine (T(4)) concentrations were determined simultaneously using liquid chromatography tandem mass-spectrometry (LC/MS/MS). Free thyroxine (FT(4)), thyroid-stimulating hormone (TSH), and thyroglobulin (Tg) were measured using immunoassay techniques. Linear mixed effects models and correlations were calculated to determine trends and associations, respectively, in concentrations.

RESULTS AND CONCLUSIONS

Trimester-specific T(3), FT(4), TSH, and Tg concentrations were significantly different between the first and third trimesters (all p < 0.05); second and third trimester values were not significantly different for FT(4), TSH, and Tg (all p > 0.25) although T3 was significantly higher in the third, relative to the second trimester. T(4) was not significantly different at any trimester (all p > 0.80). With two exceptions, analyte concentrations tended not to be correlated at each trimester and at 1-year postpartum. One exception was that T(3) and T(4) tended to be associated (all p < 0.05) at all time points except the third trimester (rho = 0.239, p > 0.05). T(4) and FT(4) concentrations tended to correlate positively during pregnancy (rho 0.361-0.382, all p < 0.05) but not postpartum (rho = 0.179, p > 0.05). Trends suggest that trimester-specific measurements of T(3), FT(4), Tg, and possibly TSH are warranted.