CCA-13340; No of Pages 5 Clinica Chimica Acta xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Clinica Chimica Acta journal homepage: www.elsevier.com/locate/clinchim
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Jonathan P. Bestwick a, Rhys John b, Aldo Maina c, Varvara Guaraldo c, Mohammed Joomun a, Nicholas J. Wald a, John H. Lazarus d,⁎
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Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs)
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Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK Department of Medical Biochemistry, University Hospital of Wales, Cardiff, UK Azienda Ospedaliera, Ospedale Infantile Regina Margherita, Sant'Anna, Turin, Italy d Thyroid Research Group, Institute of Molecular Medicine Cardiff University School of Medicine, Cardiff, UK b
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Background: Thyroid stimulating hormone (TSH) and free thyroxine (FT4) concentrations vary during pregnancy and conventional units can vary between laboratories. Reference ranges are widely quoted but are arbitrary and do not allow for inter-laboratory differences or gestational age. We therefore explored using multiple of the median (MoM) values to overcome these limitations. Methods: TSH and FT4 concentrations from 16,346 UK and 5500 Italian women less than 16 weeks of gestation collected as part of the CATS study were converted into MoMs. Effects of maternal age, gestational age, maternal weight, smoking, parity and season of blood sampling were analysed and values adjusted for influencing factors. Distributions of adjusted MoMs were determined. Results: TSH and FT4 (MoMs) significantly reduced the difference between UK and Italian samples (FT4 N TSH) compared with conventional units. TSH and FT4 MoMs were statistically significantly influenced by weight, smoking and parity; season also influenced TSH and age influenced FT4. The first and 99th centile MoMs were TSH 0.2 and 4.01, and FT4 0.75 and 1.39. Conclusion: Use of TSH and FT4 MoMs in early pregnancy allows for systematic differences between laboratories and other factors. Their use indicates high or low levels in a quantitative manner independent of reference ranges. © 2013 Published by Elsevier B.V.
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Article history: Received 29 November 2013 Received in revised form 20 December 2013 Accepted 20 December 2013 Available online xxxx
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Keywords: TSH Free T4 Pregnancy Multiple of the median MoM
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1. Introduction
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There are significant changes in thyroid physiology and function during pregnancy [1]. These are particularly important in the first trimester when the foetus relies on circulating maternal thyroxine (T4). During this time maternal T4 enters the foetus via the placenta [2] and is an important factor in foetal nervous system development [3]. Maternal thyroid function during this period is characterised by a decrease in thyroid stimulating hormone (TSH) in response to the increased concentration of human chorionic gonadotrophin [4]. There is a steady increase in maternal total T4 concentration up to approximately the end of the first trimester [4]. Free T4 (FT4) decreases between the first and second trimesters but there are limited data on the precise change between about 8 and 15 weeks [5,6]. It is recommended that each laboratory produces its own reference range to assess whether TSH and FT4 concentrations are abnormal as there are many different thyroid hormone assays which yield different results in pregnancy [7]. Reference ranges however, do not indicate
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⁎ Corresponding author. E-mail address: [email protected] (J.H. Lazarus).
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how a particular concentration is likely to be associated with clinical disease [8,9]. To do this the distributions of concentrations in affected and unaffected individuals need to be specified from which the risk of having a thyroid disorder given a particular value of a thyroid hormone can be calculated, as is done in antenatal screening for Down's syndrome. If the risk is above a specified cut-off, further action could then be taken. The definition of a thyroid disorder tends to be based on the hormone level itself making it difficult to identify a group of people with the disorder and a group without from which to independently derive separate distributions for thyroid hormones. Using reference ranges that arbitrarily identify the top 2.5% or bottom 2.5% of people as having thyroid dysfunction can be misleading and may lead to unnecessary treatment for many people. If, instead of using an arbitrary reference range, the distribution of each thyroid hormone were specified clinicians could determine on which centile a patients' result lay. It may be appropriate to express thyroid hormone levels as multiple of the median (MoM) values, as is done for antenatal Down's syndrome screening markers; this would allow for inter-laboratory assay variation, gestational age and other factors that influence marker levels. In the Controlled Antenatal Thyroid Screening Study (CATS) [10] nearly 22,000 blood samples were obtained between 8 weeks and
0009-8981/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.cca.2013.12.030
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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The Controlled Antenatal Thyroid Screening Study was performed to assess the effects on cognitive function at 3 years of age in the offspring of women who underwent thyroid screening in early pregnancy and received treatment if they had a high TSH level, a low FT4 level, or both [8]. In brief, pregnant women were invited to participate at their first antenatal hospital visit from 10 UK and 1 Italian centres. Blood samples were provided from women who were aged 18 or older, had a gestational age of less than 16 weeks 0 day, had singleton pregnancies, had no known thyroid disease and consented to participate (16,346 women from the UK and 5500 from Italy). On receipt of blood samples at the University Hospital of Wales, Cardiff, or at Ospedale Sant'Anna, Turin, Italy, women were randomly assigned with the use of a computer-generated block design to a screening or control group.
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2.2. TSH and FT4 measurements
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Serum samples from the screening group were immediately assayed for levels of FT4 and TSH. Serum samples from women in the control group were stored at − 40 °C and assayed for levels of FT4 and TSH after delivery. In the United Kingdom, levels of serum TSH and FT4 were measured with the use of immunochemiluminescence (ADVIA Centaur, Siemens Healthcare Diagnostics). In Turin, TSH and FT4 were measured with the use of an immunofluorescence method (AutoDELFIA, PerkinElmer Life and Analytical Sciences).
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2.3. Statistical analysis
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The distributions of TSH and FT4 concentrations differed between UK and Italian women and the gestational age range was wider in UK women than in Italian women [8] so concentrations were converted into MoM values, allowing for gestational age, separately for UK and Italian women. This was done by estimating the expected TSH and FT4 concentrations according to gestational age using linear regression, separately for UK and Italian women. MoM values were then calculated as the observed concentration divided by the expected median for the same gestational age. UK and Italian data were then combined and univariate regression analyses were used to determine if maternal age and maternal weight were associated with TSH and FT4 MoM values. Wilcoxon rank sum tests were used to determine if parity, smoking status and season of blood sampling were associated with TSH and FT4 MoM values. Multivariate regression was then used to determine which combinations of these factors were predictors of TSH or FT4 MoM values. Adjusted TSH and FT4 MoM values were then calculated by dividing the MoM values by those expected from the multivariate regression analyses. The distribution of adjusted MoM values was then specified and selected centiles of the MoM distributions calculated by counting. The correlation between TSH and FT4 MoM values was calculated. All analyses were performed in Stata version 12 (StatCorp, College Station, Texas).
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3. Results
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3.1. Patient characteristics
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Table 1 shows the characteristics of the CATS population. Median TSH was 1.11 mIU/L in UK women and 1.07 mIU/L in Italian women (p b 0.001). Median FT4 was 13.9 pmol/L in UK women and 9.3 pmol/L in Italian women (p b 0.001). TSH values were negatively skewed so a
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Fig. 1 shows that with increasing gestational age TSH concentrations (square root) increase but FT4 concentrations (log10) decrease linearly, confirming results from previous studies. Data from the UK women is more informative because of the wider gestational age range (7–
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3.2. Factors associated with TSH and FT4
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square root-transformation was used; FT4 values were positively skewed so a log-transformation was used. Supplemental Figures 1 and 2 show the histograms and probability plots for TSH and FT4 in UK and Italian women respectively.
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5505 1.07 (0.66–1.60) 9.3 (8.6–10.0) 12,1 (11,6–12,3) 32 (30–35) 59 (54–65) 8% 47%
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UK women
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N TSH (mIU/L) FT4 (pmol/L) Gestational age (weeks, days) Maternal age (years) Maternal weight (kg) Smoker Parous Month of blood sampling December–February March–May June–August September–November
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2. Materials and methods
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Table 1 t1:1 Characteristics of the CATS population (figures are median and interquartile range unless t1:2 otherwise stated). t1:3
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15 weeks and 6 days and were analysed for FT4 and TSH. Based on these data we here show the calculated distributions of TSH and FT4 in observed levels and MoM values.
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Fig. 1. TSH ((A), mIU/L) and FT4 ((B), pmol/L) median concentrations (with 95% confidence intervals) according to week of gestation in UK (open circles) and Italian (solid squares) women together with regression line for UK women (based on all data points) and number of UK and Italian women (median and confidence intervals not shown for weeks with few Italian women).
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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mIU/L
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Italy
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0.02 0.04 0.06 0.16 0.31 1 2.06 2.52 3.15 3.38 4.13
0.01 0.02 0.04 0.14 0.31 1 2.05 2.55 3.04 3.18 3.93
10.3 10.8 10.9 11.4 11.9 13.9 16.2 17.0 17.9 18.1 19.3
7.0 7.3 7.4 7.7 8.0 9.3 10.8 11.4 12.2 12.5 13.6
0.75 0.78 0.79 0.82 0.86 1 1.16 1.22 1.28 1.30 1.38
0.76 0.79 0.79 0.83 0.86 1 1.16 1.23 1.32 1.34 1.46
3.3. Distribution of TSH and FT4 MoM values
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16 weeks): TSH concentration decreased by 0.5 mmol/L per week of gestation (p b 0.001) and FT4 concentration decreased by 1.3% per week. Table 2 shows selected centiles of TSH and FT4 in conventional units and MoM values allowing for gestational age (separately for UK and Italian women). After converting into MoM values there was no statistically significant difference in the distributions of TSH and FT4 between UK and Italian women (p = 0.372 and p = 0.987 respectively) and centile values were similar. For example, the 90th centile of FT4 was 1.16 MoM in both UK women and Italian
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Fig. 3 shows the distribution and selected centile of TSH and FT4 180 MoM values, adjusted for the factors described above and given in 181 Table 4. The 1st and 99th centiles of TSH MoM values were 0.02 and 182 Q4
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MoM
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pmol/L
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1 2 2.5 5 10 50 90 95 97.5 98 99
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women but, in conventional units it was 16.2 pmol/L in UK women and 10.8 pmol/L in Italian women. Fig. 2 shows that TSH MoM values show no material change with increasing maternal age but increase significantly with maternal weight by 0.025 MoM per 10 kg increase (p b 0.001). FT4 MoM values decreased with both maternal age and maternal weight, 0.6% MoM for every five years of maternal age (p b 0.001) and by 0.9% MoM for every 10 kg increase in maternal weight (p b 0.001). Table 3 shows the effect of smoking, parity and season of blood sampling on TSH and FT4 MoM values. TSH and FT4 MoM values were marginally lower in smokers than non-smokers (TSH about 5% lower, FT4 2% lower), lower in parous than nulliparous women (TSH about 10% lower, FT4 1% lower) and TSH values were higher in winter months at the time of blood sampling (about 10% higher in winter than in summer) (all p b 0.001). Median FT4 MoM values were similar in different seasons although there was a statistically significant difference in the distributions between winter and summer months (p = 0.002). In multiple regression analyses maternal weight, smoking status, parity and season of blood sampling remained associated with TSH, and maternal age, maternal weight, smoking status and parity remained associated with FT4 (see Supplemental Table 1).
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Table 2 Selected centiles of TSH and FT4 in observed units and multiple of the gestation specific median (MoM) in UK and Italian women.
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Fig. 2. TSH and FT4 median MoM values (with 95% confidence intervals) according to categories of maternal age ((A) TSH, (B) FT4) and maternal weight ((C) TSH, (D) FT4) together with regression lines.
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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0.95 1.01 b0.001 0.97 1.06 b0.001 1.05 1.02 0.96 0.98 b0.001
0.98 1.00 b0.001 1.00 1.01 b0.001 1.00 1.00 1.00 1.00 0.005
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4. Discussion
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4.1. Advantage of converting TSH and FT4 into MoM values
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Our results indicate that there is merit in converting TSH and FT4 values to laboratory gestation-specific MoMs, and also some advantage in taking account of other factors that influence these concentrations. While other studies have presented TSH and FT4 values as centiles [5,11–13] (three referred to the use of MoMs [5,12,13]) we are not aware of any study that has presented the distributions in terms of MoM values and the centile values based on the distribution of the MoMs. Our results show that it is important to specify laboratory and gestation specific medians. Adjusting for other factors reduces variance of the distributions, but only to a small extent. Ethnic status has been shown to influence the two hormone levels but this was not collected as part of the CATS study. It has been shown that ethnic status could be included if such data were available; previous work has shown that both hormones are, on average, statistically significantly lower in blacks than whites [13]. The advantage of using MoM values and reporting the MoM with the centile it corresponds to is that it does not place a woman into an abnormal category, which is the case with reference ranges. This will avoid leading to unnecessary treatment for many, for example, a proportion of the 2.5% of women with the highest TSH values. The use of MoM values also has the advantage in that they take account of systematic differences between laboratories or assays, as evident from the distributions of MoM values being similar in UK and Italian women (Table 2) whereas the conventional units were less similar for TSH and markedly different for FT4. MoM values also allow for the factors that influence TSH and FT4 values, such as gestational age and maternal weight.
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4.2. Using TSH and FT4 MoM values in practice
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Most women attend for antenatal screening for Down's syndrome in the late first and/or early second trimester of pregnancy, and screening computer software already converts Down's syndrome marker levels into MoM values, allowing for factors that affect the markers such as gestational age, maternal weight and smoking. Such software can be
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4.01 respectively; the 1st and 99th centiles of FT4 were 0.75 and 1.39. The spike on the third bar of the TSH histogram is due to the minimum reported value of TSH being higher in UK women (0.02 miU/L) than in Italian women (0.001 miU/L). Supplemental Figure 3 shows the median FT4 MoM values according to deciles of TSH MoM values. In the lowest decile of TSH MoM values the median FT4 MoM was 1.09. In the next decile the median FT4 MoM was 1.02 after which there was a steady small decrease to 0.96 MoM in the highest TSH MoM decile. The correlation coefficient between raw TSH and FT4 MoM values was −0.301 (p b 0.001).
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Smokers Non-smokers p-value Parous Nulliparous p-Value December–February March–May June–August September–November p-Value (Dec–Feb vs. Jun–Aug)
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Table 3 Effect on TSH and FT4 median multiples of the gestation specific median (MoMs) of smoking status, parity and month of blood sampling.
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Fig. 3. Distribution of TSH (A) and FT4 (B) multiple of the median (MoM) values (TSH allowing for gestational age, maternal weight, smoking and parity and month of blood sampling, FT4 allowing for gestational age, maternal age, maternal weight, smoking, and parity) with centile values. TSH histogram shown for TSH ≤ 6 MoM (62 women had TSH N 6 MoM), and FT4 histogram shown for FT4 ≤ 1.8 MoM (36 women had FT4 N 6 MoM).
adapted so that laboratories can calculate their own expected medians from which to calculate MoM values for TSH and FT4, and data from about 100 women would be sufficient to do so. As with Down's syndrome screening markers, expected medians would then be reviewed periodically and if necessary adjusted. The MoM value, together with its centile value, based on the distributions in Figs. 4 and 5 which are reliable due to the large sample size, could be reported so that clinicians could judge whether treatment is needed. If treatment were prescribed the test of whether the patient has thyroid dysfunction would be their response to treatment; if there was clear symptomatic improvement (for example the relief of depression and tiredness) treatment could be continued, but not otherwise [14]. There is evidence that very high or low values of TSH or FT4 are associated with adverse outcomes in pregnancy. Medici et al. [15] showed that maternal high-normal FT4 concentrations in early pregnancy are associated with lower birth weight and an increased risk of small for gestational age newborns and in pregnant women without a history of thyroid dysfunction, lower concentrations of FT4 were found to be associated with a less favourable metabolic phenotype and with more placental growth [16]. Pop et al. [17] have shown recently that maternal thyroid measurements are related to both prepregnancy BMI and weight gain throughout gestation.
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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5. Conclusion
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Developing evidence based MoM cut-off levels for such disorders would be worthwhile and would be a productive area for research. The purpose of this paper is to propose the approach and offer guidance on the distributions derived from nearly 22,000 pregnant women.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.cca.2013.12.030.
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[1] Krassas GE, Poppe K, Glinoer D. Thyroid function and human reproductive health. Endocr Rev 2010;31:702–55. [2] Chan SY, Vasilopoulou E, Kilby MD. The role of the placenta in thyroid hormone delivery to the fetus. Nat Clin Pract Endocrinol Metab 2009;5:45–54. [3] Morreale de Escobar G, Obregon MJ, Escobar del Rey F. Role of thyroid hormone during early brain development. Eur J Endocrinol 2004;151(Suppl. 3):U25–37. [4] Glinoer D. The regulation of thyroid function in pregnancy: pathways of endocrine adaptation from physiology to pathology. Endocr Rev 1997;18:404–33. [5] Lambert-Messerlian G, McClain M, Haddow JE, et al. First- and second-trimester thyroid hormone reference data in pregnant women: a FaSTER (First- and SecondTrimester Evaluation of Risk for aneuploidy) Research Consortium study. Am J Obstet Gynecol 2008;199:62.e1–6.
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[6] Casey BM, Dashe JS, Spong CY, McIntire DD, Leveno KJ, Cunningham GF. Perinatal significance of isolated maternal hypothyoxinemia identified in the first half of pregnancy. Obstet Gynecol 2007;109:1129–35. [7] d'Herbomez M, Forzy G, Gasser F, Massart C, Beaudonnet A, Sapin R. Clinical evaluation of nine free thyroxine assays: persistent problems in particular populations. Clin Chem Lab Med 2003;41:942–7. [8] Wald NJ, Cuckle HS, Densem JW, et al. Maternal serum screening for Down's syndrome in early pregnancy. BMJ 1988;297:883–7. [9] Wald NJ. The triple test. Clin Chem 2013 (in press). [10] Lazarus JH, Bestwick JP, Channon S, et al. Antenatal thyroid screening and childhood cognitive function. N Engl J Med 2012;9(366):493–501. [11] Haddow JE, Knight GJ, Palomaki GE, McClain MR, Pulkkinen AJ. The reference range and within-person variability of thyroid stimulating hormone during the first and second trimesters of pregnancy. J Med Screen 2004;11:170–4. [12] Dashe JS, Casey BM, Wells CE, et al. Thyroid-stimulating hormone in singleton and twin pregnancy: importance of gestational age-specific reference ranges. Obstet Gynecol 2005;106:753–7. [13] Ashoor G, Kametas NA, Akolekar R, Guisado J, Nicolaides KH. Maternal thyroid function at 11–13 weeks of gestation. Fetal Diagn Ther 2010;27(3):156–63. [14] Abu-Helalah M, Law MR, Bestwick JP, Monson JP, Wald NJ. A randomized doubleblind crossover trial to investigate the efficacy of screening for adult hypothyroidism. J Med Screen 2010;17:164–9. [15] Medici M, Timmermans S, Visser W, et al. Maternal thyroid hormone parameters during early pregnancy and birth weight: the Generation R Study. J Clin Endocrinol Metab 2013;98:59–66. [16] Bassols J, Prats-Puig A, Soriano-Rodríguez P, et al. Lower free thyroxin associates with a less favorable metabolic phenotype in healthy pregnant women. J Clin Endocrinol Metab 2011;96:3717–23. [17] Pop VJ, Biondi B, Wijnen HA, Kuppens SM, Lvader H. Maternal thyroid parameters, Body Mass Index and subsequent weight gain during pregnancy in healthy euthyroid women. Clin Endocrinol (Oxf) 2013 [Epub ahead of print].
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Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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Contents lists available at ScienceDirect
Clinica Chimica Acta journal homepage: www.elsevier.com/locate/clinchim
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Jonathan P. Bestwick a, Rhys John b, Aldo Maina c, Varvara Guaraldo c, Mohammed Joomun a, Nicholas J. Wald a, John H. Lazarus d,⁎
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Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs)
a
Wolfson Institute of Preventive Medicine, Barts and the London School of Medicine and Dentistry, Queen Mary University of London, London, UK Department of Medical Biochemistry, University Hospital of Wales, Cardiff, UK Azienda Ospedaliera, Ospedale Infantile Regina Margherita, Sant'Anna, Turin, Italy d Thyroid Research Group, Institute of Molecular Medicine Cardiff University School of Medicine, Cardiff, UK b
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a b s t r a c t
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a r t i c l e
Background: Thyroid stimulating hormone (TSH) and free thyroxine (FT4) concentrations vary during pregnancy and conventional units can vary between laboratories. Reference ranges are widely quoted but are arbitrary and do not allow for inter-laboratory differences or gestational age. We therefore explored using multiple of the median (MoM) values to overcome these limitations. Methods: TSH and FT4 concentrations from 16,346 UK and 5500 Italian women less than 16 weeks of gestation collected as part of the CATS study were converted into MoMs. Effects of maternal age, gestational age, maternal weight, smoking, parity and season of blood sampling were analysed and values adjusted for influencing factors. Distributions of adjusted MoMs were determined. Results: TSH and FT4 (MoMs) significantly reduced the difference between UK and Italian samples (FT4 N TSH) compared with conventional units. TSH and FT4 MoMs were statistically significantly influenced by weight, smoking and parity; season also influenced TSH and age influenced FT4. The first and 99th centile MoMs were TSH 0.2 and 4.01, and FT4 0.75 and 1.39. Conclusion: Use of TSH and FT4 MoMs in early pregnancy allows for systematic differences between laboratories and other factors. Their use indicates high or low levels in a quantitative manner independent of reference ranges. © 2013 Published by Elsevier B.V.
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Article history: Received 29 November 2013 Received in revised form 20 December 2013 Accepted 20 December 2013 Available online xxxx
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Keywords: TSH Free T4 Pregnancy Multiple of the median MoM
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1. Introduction
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There are significant changes in thyroid physiology and function during pregnancy [1]. These are particularly important in the first trimester when the foetus relies on circulating maternal thyroxine (T4). During this time maternal T4 enters the foetus via the placenta [2] and is an important factor in foetal nervous system development [3]. Maternal thyroid function during this period is characterised by a decrease in thyroid stimulating hormone (TSH) in response to the increased concentration of human chorionic gonadotrophin [4]. There is a steady increase in maternal total T4 concentration up to approximately the end of the first trimester [4]. Free T4 (FT4) decreases between the first and second trimesters but there are limited data on the precise change between about 8 and 15 weeks [5,6]. It is recommended that each laboratory produces its own reference range to assess whether TSH and FT4 concentrations are abnormal as there are many different thyroid hormone assays which yield different results in pregnancy [7]. Reference ranges however, do not indicate
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⁎ Corresponding author. E-mail address: [email protected] (J.H. Lazarus).
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how a particular concentration is likely to be associated with clinical disease [8,9]. To do this the distributions of concentrations in affected and unaffected individuals need to be specified from which the risk of having a thyroid disorder given a particular value of a thyroid hormone can be calculated, as is done in antenatal screening for Down's syndrome. If the risk is above a specified cut-off, further action could then be taken. The definition of a thyroid disorder tends to be based on the hormone level itself making it difficult to identify a group of people with the disorder and a group without from which to independently derive separate distributions for thyroid hormones. Using reference ranges that arbitrarily identify the top 2.5% or bottom 2.5% of people as having thyroid dysfunction can be misleading and may lead to unnecessary treatment for many people. If, instead of using an arbitrary reference range, the distribution of each thyroid hormone were specified clinicians could determine on which centile a patients' result lay. It may be appropriate to express thyroid hormone levels as multiple of the median (MoM) values, as is done for antenatal Down's syndrome screening markers; this would allow for inter-laboratory assay variation, gestational age and other factors that influence marker levels. In the Controlled Antenatal Thyroid Screening Study (CATS) [10] nearly 22,000 blood samples were obtained between 8 weeks and
0009-8981/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.cca.2013.12.030
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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The Controlled Antenatal Thyroid Screening Study was performed to assess the effects on cognitive function at 3 years of age in the offspring of women who underwent thyroid screening in early pregnancy and received treatment if they had a high TSH level, a low FT4 level, or both [8]. In brief, pregnant women were invited to participate at their first antenatal hospital visit from 10 UK and 1 Italian centres. Blood samples were provided from women who were aged 18 or older, had a gestational age of less than 16 weeks 0 day, had singleton pregnancies, had no known thyroid disease and consented to participate (16,346 women from the UK and 5500 from Italy). On receipt of blood samples at the University Hospital of Wales, Cardiff, or at Ospedale Sant'Anna, Turin, Italy, women were randomly assigned with the use of a computer-generated block design to a screening or control group.
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2.2. TSH and FT4 measurements
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Serum samples from the screening group were immediately assayed for levels of FT4 and TSH. Serum samples from women in the control group were stored at − 40 °C and assayed for levels of FT4 and TSH after delivery. In the United Kingdom, levels of serum TSH and FT4 were measured with the use of immunochemiluminescence (ADVIA Centaur, Siemens Healthcare Diagnostics). In Turin, TSH and FT4 were measured with the use of an immunofluorescence method (AutoDELFIA, PerkinElmer Life and Analytical Sciences).
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2.3. Statistical analysis
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The distributions of TSH and FT4 concentrations differed between UK and Italian women and the gestational age range was wider in UK women than in Italian women [8] so concentrations were converted into MoM values, allowing for gestational age, separately for UK and Italian women. This was done by estimating the expected TSH and FT4 concentrations according to gestational age using linear regression, separately for UK and Italian women. MoM values were then calculated as the observed concentration divided by the expected median for the same gestational age. UK and Italian data were then combined and univariate regression analyses were used to determine if maternal age and maternal weight were associated with TSH and FT4 MoM values. Wilcoxon rank sum tests were used to determine if parity, smoking status and season of blood sampling were associated with TSH and FT4 MoM values. Multivariate regression was then used to determine which combinations of these factors were predictors of TSH or FT4 MoM values. Adjusted TSH and FT4 MoM values were then calculated by dividing the MoM values by those expected from the multivariate regression analyses. The distribution of adjusted MoM values was then specified and selected centiles of the MoM distributions calculated by counting. The correlation between TSH and FT4 MoM values was calculated. All analyses were performed in Stata version 12 (StatCorp, College Station, Texas).
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3. Results
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3.1. Patient characteristics
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Table 1 shows the characteristics of the CATS population. Median TSH was 1.11 mIU/L in UK women and 1.07 mIU/L in Italian women (p b 0.001). Median FT4 was 13.9 pmol/L in UK women and 9.3 pmol/L in Italian women (p b 0.001). TSH values were negatively skewed so a
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Fig. 1 shows that with increasing gestational age TSH concentrations (square root) increase but FT4 concentrations (log10) decrease linearly, confirming results from previous studies. Data from the UK women is more informative because of the wider gestational age range (7–
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3.2. Factors associated with TSH and FT4
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square root-transformation was used; FT4 values were positively skewed so a log-transformation was used. Supplemental Figures 1 and 2 show the histograms and probability plots for TSH and FT4 in UK and Italian women respectively.
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27% 26% 20% 27%
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24% 26% 25% 26%
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t1:5 t1:6 t1:7 t1:8 t1:9 t1:10 t1:11 t1:12 t1:13 t1:14 t1:15 t1:16 t1:17
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t1:4
5505 1.07 (0.66–1.60) 9.3 (8.6–10.0) 12,1 (11,6–12,3) 32 (30–35) 59 (54–65) 8% 47%
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Italian women
16,334 1.11 (0.68–1.65) 13.9 (12.8–15.0) 12,4 (11,6–13,5) 29 (24–33) 67 (59–77) 21% 65%
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2.1. Patients
UK women
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N TSH (mIU/L) FT4 (pmol/L) Gestational age (weeks, days) Maternal age (years) Maternal weight (kg) Smoker Parous Month of blood sampling December–February March–May June–August September–November
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2. Materials and methods
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Table 1 t1:1 Characteristics of the CATS population (figures are median and interquartile range unless t1:2 otherwise stated). t1:3
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15 weeks and 6 days and were analysed for FT4 and TSH. Based on these data we here show the calculated distributions of TSH and FT4 in observed levels and MoM values.
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Fig. 1. TSH ((A), mIU/L) and FT4 ((B), pmol/L) median concentrations (with 95% confidence intervals) according to week of gestation in UK (open circles) and Italian (solid squares) women together with regression line for UK women (based on all data points) and number of UK and Italian women (median and confidence intervals not shown for weeks with few Italian women).
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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mIU/L
t2:6
Italy
UK
Italy
UK
Italy
0.02 0.04 0.06 0.16 0.31 1 2.06 2.52 3.15 3.38 4.13
0.01 0.02 0.04 0.14 0.31 1 2.05 2.55 3.04 3.18 3.93
10.3 10.8 10.9 11.4 11.9 13.9 16.2 17.0 17.9 18.1 19.3
7.0 7.3 7.4 7.7 8.0 9.3 10.8 11.4 12.2 12.5 13.6
0.75 0.78 0.79 0.82 0.86 1 1.16 1.22 1.28 1.30 1.38
0.76 0.79 0.79 0.83 0.86 1 1.16 1.23 1.32 1.34 1.46
3.3. Distribution of TSH and FT4 MoM values
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16 weeks): TSH concentration decreased by 0.5 mmol/L per week of gestation (p b 0.001) and FT4 concentration decreased by 1.3% per week. Table 2 shows selected centiles of TSH and FT4 in conventional units and MoM values allowing for gestational age (separately for UK and Italian women). After converting into MoM values there was no statistically significant difference in the distributions of TSH and FT4 between UK and Italian women (p = 0.372 and p = 0.987 respectively) and centile values were similar. For example, the 90th centile of FT4 was 1.16 MoM in both UK women and Italian
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Fig. 3 shows the distribution and selected centile of TSH and FT4 180 MoM values, adjusted for the factors described above and given in 181 Table 4. The 1st and 99th centiles of TSH MoM values were 0.02 and 182 Q4
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0.01 0.02 0.04 0.14 0.33 1.07 2.19 2.68 3.19 3.38 4.10
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Italy
0.02 0.04 0.06 0.18 0.34 1.11 2.30 2.82 3.50 3.75 4.57
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UK
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MoM
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pmol/L
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1 2 2.5 5 10 50 90 95 97.5 98 99
MoM
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FT4
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TSH
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women but, in conventional units it was 16.2 pmol/L in UK women and 10.8 pmol/L in Italian women. Fig. 2 shows that TSH MoM values show no material change with increasing maternal age but increase significantly with maternal weight by 0.025 MoM per 10 kg increase (p b 0.001). FT4 MoM values decreased with both maternal age and maternal weight, 0.6% MoM for every five years of maternal age (p b 0.001) and by 0.9% MoM for every 10 kg increase in maternal weight (p b 0.001). Table 3 shows the effect of smoking, parity and season of blood sampling on TSH and FT4 MoM values. TSH and FT4 MoM values were marginally lower in smokers than non-smokers (TSH about 5% lower, FT4 2% lower), lower in parous than nulliparous women (TSH about 10% lower, FT4 1% lower) and TSH values were higher in winter months at the time of blood sampling (about 10% higher in winter than in summer) (all p b 0.001). Median FT4 MoM values were similar in different seasons although there was a statistically significant difference in the distributions between winter and summer months (p = 0.002). In multiple regression analyses maternal weight, smoking status, parity and season of blood sampling remained associated with TSH, and maternal age, maternal weight, smoking status and parity remained associated with FT4 (see Supplemental Table 1).
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Table 2 Selected centiles of TSH and FT4 in observed units and multiple of the gestation specific median (MoM) in UK and Italian women.
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Fig. 2. TSH and FT4 median MoM values (with 95% confidence intervals) according to categories of maternal age ((A) TSH, (B) FT4) and maternal weight ((C) TSH, (D) FT4) together with regression lines.
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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0.95 1.01 b0.001 0.97 1.06 b0.001 1.05 1.02 0.96 0.98 b0.001
0.98 1.00 b0.001 1.00 1.01 b0.001 1.00 1.00 1.00 1.00 0.005
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4. Discussion
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4.1. Advantage of converting TSH and FT4 into MoM values
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Our results indicate that there is merit in converting TSH and FT4 values to laboratory gestation-specific MoMs, and also some advantage in taking account of other factors that influence these concentrations. While other studies have presented TSH and FT4 values as centiles [5,11–13] (three referred to the use of MoMs [5,12,13]) we are not aware of any study that has presented the distributions in terms of MoM values and the centile values based on the distribution of the MoMs. Our results show that it is important to specify laboratory and gestation specific medians. Adjusting for other factors reduces variance of the distributions, but only to a small extent. Ethnic status has been shown to influence the two hormone levels but this was not collected as part of the CATS study. It has been shown that ethnic status could be included if such data were available; previous work has shown that both hormones are, on average, statistically significantly lower in blacks than whites [13]. The advantage of using MoM values and reporting the MoM with the centile it corresponds to is that it does not place a woman into an abnormal category, which is the case with reference ranges. This will avoid leading to unnecessary treatment for many, for example, a proportion of the 2.5% of women with the highest TSH values. The use of MoM values also has the advantage in that they take account of systematic differences between laboratories or assays, as evident from the distributions of MoM values being similar in UK and Italian women (Table 2) whereas the conventional units were less similar for TSH and markedly different for FT4. MoM values also allow for the factors that influence TSH and FT4 values, such as gestational age and maternal weight.
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4.2. Using TSH and FT4 MoM values in practice
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Most women attend for antenatal screening for Down's syndrome in the late first and/or early second trimester of pregnancy, and screening computer software already converts Down's syndrome marker levels into MoM values, allowing for factors that affect the markers such as gestational age, maternal weight and smoking. Such software can be
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4.01 respectively; the 1st and 99th centiles of FT4 were 0.75 and 1.39. The spike on the third bar of the TSH histogram is due to the minimum reported value of TSH being higher in UK women (0.02 miU/L) than in Italian women (0.001 miU/L). Supplemental Figure 3 shows the median FT4 MoM values according to deciles of TSH MoM values. In the lowest decile of TSH MoM values the median FT4 MoM was 1.09. In the next decile the median FT4 MoM was 1.02 after which there was a steady small decrease to 0.96 MoM in the highest TSH MoM decile. The correlation coefficient between raw TSH and FT4 MoM values was −0.301 (p b 0.001).
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Smokers Non-smokers p-value Parous Nulliparous p-Value December–February March–May June–August September–November p-Value (Dec–Feb vs. Jun–Aug)
Median FT4 (MoM)
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t3:5 t3:6 t3:7 t3:8 t3:9 t3:10 t3:11 t3:12 t3:13 t3:14 t3:15
Median TSH (MoM)
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t3:4
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Table 3 Effect on TSH and FT4 median multiples of the gestation specific median (MoMs) of smoking status, parity and month of blood sampling.
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Fig. 3. Distribution of TSH (A) and FT4 (B) multiple of the median (MoM) values (TSH allowing for gestational age, maternal weight, smoking and parity and month of blood sampling, FT4 allowing for gestational age, maternal age, maternal weight, smoking, and parity) with centile values. TSH histogram shown for TSH ≤ 6 MoM (62 women had TSH N 6 MoM), and FT4 histogram shown for FT4 ≤ 1.8 MoM (36 women had FT4 N 6 MoM).
adapted so that laboratories can calculate their own expected medians from which to calculate MoM values for TSH and FT4, and data from about 100 women would be sufficient to do so. As with Down's syndrome screening markers, expected medians would then be reviewed periodically and if necessary adjusted. The MoM value, together with its centile value, based on the distributions in Figs. 4 and 5 which are reliable due to the large sample size, could be reported so that clinicians could judge whether treatment is needed. If treatment were prescribed the test of whether the patient has thyroid dysfunction would be their response to treatment; if there was clear symptomatic improvement (for example the relief of depression and tiredness) treatment could be continued, but not otherwise [14]. There is evidence that very high or low values of TSH or FT4 are associated with adverse outcomes in pregnancy. Medici et al. [15] showed that maternal high-normal FT4 concentrations in early pregnancy are associated with lower birth weight and an increased risk of small for gestational age newborns and in pregnant women without a history of thyroid dysfunction, lower concentrations of FT4 were found to be associated with a less favourable metabolic phenotype and with more placental growth [16]. Pop et al. [17] have shown recently that maternal thyroid measurements are related to both prepregnancy BMI and weight gain throughout gestation.
Please cite this article as: Bestwick JP, et al, Thyroid stimulating hormone and free thyroxine in pregnancy: Expressing concentrations as multiples of the median (MoMs), Clin Chim Acta (2013), http://dx.doi.org/10.1016/j.cca.2013.12.030
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5. Conclusion
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Developing evidence based MoM cut-off levels for such disorders would be worthwhile and would be a productive area for research. The purpose of this paper is to propose the approach and offer guidance on the distributions derived from nearly 22,000 pregnant women.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.cca.2013.12.030.
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