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Initial evaluation of thyroid dysfunction - Are simultaneous TSH and fT4 tests necessary?

  • Claudio Schneider,

    Roles Conceptualization, Investigation, Methodology, Writing – original draft, Writing – review & editing

    Affiliation Department of General Internal Medicine, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland

  • Martin Feller,

    Roles Investigation, Supervision, Writing – review & editing

    Affiliations Department of General Internal Medicine, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland, Institute of Primary Health Care (BIHAM), University of Bern, Bern, Switzerland

  • Douglas C. Bauer,

    Roles Investigation, Writing – review & editing

    Affiliation Departments of Medicine and Epidemiology and Biostatistics, University of California, San Francisco, California, United States

  • Tinh-Hai Collet,

    Roles Investigation, Writing – review & editing

    Affiliation Service of Endocrinology, Diabetes and Metabolism, Lausanne University Hospital, Lausanne, Switzerland

  • Bruno R. da Costa,

    Roles Formal analysis, Writing – review & editing

    Affiliations Service of Endocrinology, Diabetes and Metabolism, Lausanne University Hospital, Lausanne, Switzerland, Applied Health Research Centre, Li Ka Shing Knowledge Institute of St Michael's Hospital, Institute of Health Policy, Management and Evaluation, University of Toronto, Toronto, Canada

  • Reto Auer,

    Roles Conceptualization, Writing – review & editing

    Affiliation Institute of Primary Health Care (BIHAM), University of Bern, Bern, Switzerland

  • Robin P. Peeters,

    Roles Writing – review & editing

    Affiliation Department of Internal Medicine, Erasmus Medical Center, Rotterdam, the Netherlands

  • Suzanne J. Brown,

    Roles Data curation, Writing – review & editing

    Affiliation Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia

  • Alexandra P. Bremner,

    Roles Data curation, Writing – review & editing

    Affiliation School of Population Health, The University of Western Australia, Crawley, Western Australia, Australia

  • Peter C. O’Leary,

    Roles Data curation, Writing – review & editing

    Affiliation School of Women's and Infants' Health, The University of Western Australia, Crawley, Western Australia, Australia

  • Peter Feddema,

    Roles Data curation, Writing – review & editing

    Affiliation Diagnostica Stago, Doncaster, Victoria, Australia

  • Peter J. Leedman,

    Roles Data curation, Writing – review & editing

    Affiliations Medical School, University of Western Australia, Crawley, Western Australia, Australia, UWA Centre for Medical Research, Harry Perkins Institute for Medical Research, Perth, Western Australia, Australia

  • Drahomir Aujesky,

    Roles Supervision, Writing – review & editing

    Affiliation Department of General Internal Medicine, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland

  • John P. Walsh,

    Roles Conceptualization, Supervision, Writing – review & editing

    Affiliations Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia, Medical School, University of Western Australia, Crawley, Western Australia, Australia

  • Nicolas Rodondi

    Roles Conceptualization, Methodology, Supervision, Writing – review & editing

    nicolas.rodondi@insel.ch

    Affiliations Department of General Internal Medicine, Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland, Institute of Primary Health Care (BIHAM), University of Bern, Bern, Switzerland

Initial evaluation of thyroid dysfunction - Are simultaneous TSH and fT4 tests necessary?

  • Claudio Schneider, 
  • Martin Feller, 
  • Douglas C. Bauer, 
  • Tinh-Hai Collet, 
  • Bruno R. da Costa, 
  • Reto Auer, 
  • Robin P. Peeters, 
  • Suzanne J. Brown, 
  • Alexandra P. Bremner, 
  • Peter C. O’Leary
PLOS
x

Abstract

Objective

Guidelines for thyroid function evaluation recommend testing TSH first, then assessing fT4 only if TSH is out of the reference range (two-step), but many clinicians initially request both TSH and fT4 (one-step). Given limitations of previous studies, we aimed to compare the two-step with the one-step approach in an unselected community-dwelling study population, and develop a prediction score based on clinical parameters that could identify at-risk patients for thyroid dysfunction.

Design

Cross-sectional analysis of the population-based Busselton Health Study.

Methods

We compared the two-step with the one-step approach, focusing on cases that would be missed by the two-step approach, i.e. those with normal TSH, but out-of-range fT4. We used likelihood ratio tests to identify demographic and clinical parameters associated with thyroid dysfunction and developed a clinical prediction score by using a beta-coefficient based scoring method.

Results

Following the two-step approach, 93.0% of all 4471 participants had normal TSH and would not need further testing. The two-step approach would have missed 3.8% of all participants (169 of 4471) with a normal TSH, but a fT4 outside the reference range. In 85% (144 of 169) of these cases, fT4 fell within 2 pmol/l of fT4 reference range limits, consistent with healthy outliers. The clinical prediction score that performed best excluded only 22.5% of participants from TSH testing.

Conclusion

The two-step approach may avoid measuring fT4 in as many as 93% of individuals with a very small risk of missing thyroid dysfunction. Our findings do not support the simultaneous initial measurement of both TSH and fT4.

Introduction

Thyroid dysfunction is common, particularly in the elderly.[13] Elevated levels of thyroid-stimulating hormone (TSH) define hypothyroidism, and decreased TSH levels define hyperthyroidism.[47] When TSH levels are out of the reference range, but concentrations of free thyroxine (fT4) fall within the reference range,[8] this is referred to as subclinical thyroid dysfunction. This entity is defined with laboratory testing because thyroid disorders have a wide range of unspecific clinical manifestations.[9]

Thyroid function tests are among the most frequently requested laboratory procedures[10] and are often considered part of a standard work-up in many situations, such as symptoms like fatigue, anemia, confusion or palpitations[11] as well as for regular check-up examination.[12] Since the diagnosis of thyroid dysfunction requires both TSH and fT4 measurements, measuring both at once may seem most efficient (one-step approach). However, TSH is a sensitive marker of thyroid dysfunction, since small changes in fT4 create large changes in TSH.[13] Earlier studies[14, 15] of outpatients in whom pituitary/hypothalamic disease was not expected showed that it was usually unnecessary to measure fT4 when TSH was normal. Most guidelines therefore recommend a two-step approach (the thyroid cascade): TSH should be measured first, then fT4 only if TSH is out of reference range or there is suspicion of abnormal TSH secretion.[12, 14, 1618]

Despite clear recommendations,[12, 14, 1618] clinical practice varies greatly, even among experts and national thyroid associations.[19] A United States study of outpatients showed that about half of TSH measurements (47%) were accompanied by an fT4 or total thyroxine (TT4) test.[20] In their 2013 Atlas, the National Health Service of England also published data that showed 46% of TSH measurements were accompanied by an fT4 measurement.[21] Private labs may charge as much as 129 USD to assess fT4,[22] posing an unnecessary burden on patients and health care systems. This burden could drop with the consistent adoption of a two-step approach.[20] We hypothesise that clinicians do not use the two-step approach because they are concerned about missing those with normal TSH but abnormal fT4.

The need for TSH and fT4 testing could be greatly reduced if we were able to identify patients at risk for thyroid dysfunction prior to the first TSH measurement. To identify healthy participants, a clinical prediction score could help. Therefore, we set out to (i) substantiate the claim that the two-step was better than the one-step approach (for the first time in an unselected, community-dwelling study population), and (ii) develop a prediction score based on clinical parameters, to identify patients at risk for thyroid dysfunction, which could provide a novel three-step approach (first prediction score, then TSH, then fT4).

However, all previous studies have been on selected patients and the validity and safety of the two-step approach has never been examined in an unselected, community-dwelling study population.[15, 23, 24] For the first time we compared the one and the two step approaches in an unselected population in a community setting. Furthermore, as there have been no attempts to develop a clinical risk prediction score in a population-based sample, we tried to find a clinical useful prediction score to predict hypo- or hyperthyroidism.

Materials and methods

Setting and study population

We conducted a retrospective, cross-sectional analysis of 4843 participants (aged 17 to 89 years) in the 1994 Busselton Health Survey. Busselton is a rural town in Western Australia with a predominantly white, iodine-sufficient population(http://bpmri.org.au). The busselton health survey has been approved by the Human Research Ethics Committee of the Department of Health of Western Australia. All participants gave informed consent to participate in the busslton health surveys. We excluded participants on anti-thyroid medication (5 participants), on levothyroxine (128 participants), whose TSH (7 participants) or fT4 tests were missing (241 participants). A total of 4471 participants were included in the analysis.

Definition of main exposures, potential confounders and outcomes

Euthyroidism was defined as TSH within the reference range from 0.45 to 4.49 mU/L. Subclinical hypothyroidism was defined as TSH between 4.50 and 19.99 mU/L and fT4 within the reference range. Overt hypothyroidism was defined as TSH ≥ 20 mU/L or TSH between 4.50 and 19.99 mU/L and fT4 below the reference range.[25, 26] Hyperthyroidism was defined as TSH under 0.45 mU/L, and fT4 measurements allowed us to differentiate between overt hyperthyroidism (fT4 above the reference level) and subclinical hyperthyroidism (fT4 within the reference range). All blood samples were collected and frozen in 1994. Measurements were performed on a single platform in three runs (2007, 2011, and 2013), with reagents from a single supplier. The same TSH assay was used consistently throughout.[27] We defined the reference range for TSH (from 0.45 to 4.49 mU/L), based on expert reviews[8, 28] and a 2010 consensus meeting within our consortium (International Thyroid Conference, Paris, France, 2010).[26] Because the fT4 method was re-standardized by the manufacturer between 2007 and 2011, we converted the 2007 measures so they aligned with the 2011 and 2013 measures.[29] For fT4, reference ranges in the Busselton cohort were defined as 11.2 to 20.8 pmol/l.[29]

To determine if the two-step approach was as accurate and as safe as the one-step approach, we first applied the two-step approach, i.e. evaluated TSH only. Those with an abnormal TSH would subsequently have an fT4 assessment, allowing classification of thyroid dysfunction (subclinical vs overt). We then analysed the participants that would have been missed with a two-step approach, i.e. participants with a normal TSH, but an abnormal fT4. We named these conditions either isolated hypo-thyroxinemia or isolated hyper-thyroxinemia and analyzed the distribution of the fT4 values within these groups.

Assessment of a three-step approach with a clinical prediction score

We developed two clinical risk scores to predict either hypo- or hyperthyroidism (subclinical and overt). All predefined clinical and laboratory parameters of the dataset which could be associated with hypo- or hyperthyroidism were considered potential candidates for inclusion in the analysis. We chose some predictors because previous studies showed clear association between the predictor and thyroid dysfunction (e.g., smoking status,[30] body mass index (BMI),[31] and blood pressure[32]); others were chosen because of their pathophysiological plausibility. We used univariate logistic regression analysis to assess their association with hypo- or hyperthyroidism and judged corresponding p-values significant when they were < 0.05 in likelihood ratio tests. We then conducted a backward stepwise selection (significance p < 0.20) to determine the final variables to include in the prediction model, along with their respective coefficient. With the final multivariable regression models for hypo- or hyperthyroidism we developed two clinical risk scores, by applying a regression beta-coefficient based scoring method.[33] Integer scores were assigned by dividing risk-factor beta-coefficients by the smallest coefficient and rounding up to the nearest integer.

Results

Of the 4471 participants included in the analysis, a total of 4156 (93%) had a normal TSH, 35 (0.8%) overt hypothyroidism, 86 (1.9%) subclinical hypothyroidism, 170 (3.8%) subclinical hyperthyroidism, and 23 (0.5%) overt hyperthyroidism. One participant’s TSH and fT4 were both below reference range. He was excluded because of potential euthyroid sick syndrome or hypopituitarism. Mean age was 51.1 (age range 16.5–97.3 years); 55.2% were women (Table 1).

Accuracy of one and two-step approach

Among the 4471 participants, 4156 (93%) had a TSH within the reference range, 3987 (89.2%) had both TSH and fT4 within the reference range. This implies that 96% (3987 / 4156) of the participants with a TSH within the reference are correctly diagnosed as euthyroid by the two-step approach without measuring the fT4. In 169 (3.8%) participants TSH was normal but fT4 was outside the reference range, they comprised 82 participants with isolated hypo-thyroxinemia (normal TSH, decreased fT4) and 87 participants with isolated hyper-thyroxinemia (normal TSH, elevated fT4)(Table 2). Using the two-step approach, these patients would have remained undetected because their TSH was within the reference range. In most, however (85% of the 3.8%) fT4 concentrations were within 2 pmol/l of the reference range limits of fT4 (Fig 1 dashed line). For the remaining 315 (7%) participants with an abnormal TSH, their fT4 would be assessed in any case in the two-step approach.

thumbnail
Fig 1. Ft4 measures when TSH falls within the normal range.

FT4 measures when TSH falls within the normal range (0.45–4.5 mU/L); 85% of abnormal fT4 measures are within 2 pmol/l (dashed lines) of the reference range (11.2 to 20.8 pmol/L, solid lines). Abbreviations: Free Thyroxine (fT4) Thyroid-Stimulating Hormone (TSH).

https://doi.org/10.1371/journal.pone.0196631.g001

Three-step approach with clinical score development and validation

Univariate likelihood ratio tests to select predictor variables for the multivariate model revealed several that were significantly associated with hypothyroidism (S2 Table). Association with age (p < 0.001) was the most significant. Sex (p = 0.002), height (p = 0.015), and BMI (p = 0.037) were also significantly associated. Predictor variables that were close to statistical significance and included in the prediction model development were systolic blood pressure (p = 0.079), alcohol consumption (p = 0.112), thyroid-affecting medication (p = 0.118, anti-thyroid medication and levothyroxine excluded) and smoking status (p = 0.129). Menopausal state (p < 0.001) was confounded by sex and age, and the association disappeared in multivariate models. Our best model for predicting hypothyroidism only explained about 5 percent of the variation (pseudo R-squared: 0.045). We included the following predictors in the model (with their coefficient in brackets): Age between 50 and 75 years (2 points), older than 75 years (4 points), female (1 point), and BMI ≥ 30 kg/m2 (1 point) (S4 Table). The Predictor Score produced a relatively poor area under the curve (AUC) of 0.67 (S1 Fig).

For hyperthyroidism, only smoking status (p = 0.041) was significantly associated with hyperthyroidism (S3 Table), so developing a clinical risk score to detect hyperthyroidism was not feasible.

Discussion

In this large unselected, community-dwelling population-based study, we showed that the two-step approach (assessing fT4 only if TSH is outside the reference range) could eliminate unnecessary fT4 testing in up to 93% of participants compared with a one-step approach. We could not identify parameters that would allow us to lower the number of TSH measurements by devising a new clinical score that predicts thyroid dysfunction in a clinically useful way.

Clinicians are concerned that the two-step approach may miss some patients with thyroid dysfunction because their TSH value was normal. First, 93.0% of all our study participants had normal TSH; 89.2% had both normal TSH and normal fT4. Thus, 95.9% (89.2% / 93.0%) of those with normal TSH also had normal fT4 levels, and are clearly euthyroid. This proportion matches the three other studies on the topic, which reported proportions of 90.5%,[24] 98.1%,[23] and 99.6%.[15] The high level of agreement across the three studies is reassuring because each included different populations. Ours consisted of unselected, healthy community-dwelling participants, Bauer et al.[23] as well as Viera et al.[15] analyzed patients from an ambulatory care center, and Kende et al.[24] analyzed hospitalized patients. Second, we analyzed patients with normal TSH and fT4 outside the reference range (3.8% of the whole study population) to determine if a sole, normal TSH assessment is likely to miss thyroid dysfunction. We found that most participants (85%) were within 2 pmol/l of the limits of the fT4 reference range. We consider these participants likely to be euthyroid, healthy outliers (the tails of a normal fT4 distribution, S2 Fig). Our interpretation is supported by Bauer et al., who reviewed the charts of those in their hospital-based study population (1.9%) whose TSH value was normal, but fT4 value was abnormal.[23] In no case did measuring fT4 lead to a new diagnosis or change the treatment of a patient. The authors concluded that none of these patients had true thyroid dysfunction.[23] Taken together, ours and previous data show that a sole, normal TSH test reliably diagnoses euthyroidism and does not miss patients with clinically relevant thyroid dysfunction.

Still, despite existing guidelines recommending the two-step approach,[12, 14, 1618] physicians vary in their approaches to thyroid testing: for example, in Great Britain the rate of test requests varies six-fold between general practitioners.[34] A 2003 study of six French hospitals showed only 62% of indicated thyroid function tests followed published guidelines.[35] Recent research in Wales showed that every third hospital admission gets a thyroid profile.[36] It remains speculative why a significant proportion of physicians still prefers the one-step approach: they may find it more convenient, consider the additional costs of simultaneous fT4 testing negligible, and/or fear to miss relevant thyroid diseases with the two-step approach. There are examples, however, where the two-step approach is well established: In New Zealand, the number of unnecessary thyroid function tests dropped from 56.8% to 34.8% in primary care.[11, 37], In France, a pre-/post-interventional study that required justification for all laboratory tests increased awareness of thyroid function testing and consecutively decreased simultaneous TSH and FT4 testing from 77% to 51%.[10] Moreover, depending on the local lab habits, biobanking of remaining serum may allow the additional analysis of fT4 from the same blood sample to be ordered only once the abnormal TSH value is known. We assessed TSH in the whole study population (n = 4471) and found that 7% had abnormal TSH values. To reduce the high proportion of unnecessary TSH tests, we tried to develop a score to predict hypo- and hyperthyroidism. The prediction scores we developed could not reliably identify healthy people who don’t need a TSH test. Since both overt and subclinical thyroid dysfunction are associated with adverse health outcomes like coronary heart disease,[26] heart failure,[38] and fractures,[25] we designed our scores to predict combined overt and subclinical thyroid dysfunction. It is possible that the prediction scores we developed did not perform well because combined subclinical and overt thyroid dysfunction reduced prediction. Collecting information about clinical symptoms might have improved our prediction scores, although reported symptoms with subclinical hypothyroidism are not specific.[3, 39] Prior studies yielded conflicting data on this issue.[9, 4043] Only two studies reported their score’s discriminative ability by measuring the AUC. Canaris et al. reported a poor AUC of 0.64 for their weighted symptom score.[9] Carlé et al’s score effectively identified overt hypothyroidism among young patients (younger men AUC of 0.91, younger women AUC of 0.84), but discriminated poorly for woman above sixty years (AUC 0.64).[41]

The strengths of this analysis lie in the unselected population and the large age range of community-dwelling individuals, which reduces the risk of selection bias and increases the generalisability of our results, at least to white, community-dwelling populations. Furthermore, we added to the body of evidence that shows an initial sole TSH test is sufficient, and fT4 testing is only necessary in a large community-dwelling population if TSH is abnormal. Our study has also limitations. First, our cohort consisted of mostly white, community-dwelling adults, so our results may not apply to all populations. Second, some patients might have been recovering from an acute non-thyroidal illness, leading to the so-called euthyroid sick syndrome or low T3 syndrome.[44] This should affect only a small part of our population, since they were all community-dwelling adults who had to attend a visit at the study centre. Third, the prevalence of subclinical hypothyroidism was somewhat lower, and that of subclinical hyperthyroidism higher than reported previously in this population.[27] This probably reflects the exclusion of individuals on thyroid medication from the current analysis, and the application of a slightly different TSH reference range based on recent publications.[25] Fourth, the two-step approach could have missed central hypothyroidism or hyperthyroidism, or impaired sensitivity to thyroid hormone.[45] In addition to these rare conditions, frequent situations of discordant thyroid function tests include non-thyroidal illness, assay and drug interference.[46] However, most of these clinical situations are accompanied by key clinical symptoms and medical history;[4749] in such patients, the threshold for fT4 measurement should be low. Fifth, we had no information about clinical symptoms in our study population. We defined subclinical hypothyroidism as TSH between 4.50 and 19.99 mU/L and fT4 within reference range. This is a widely accepted definition of subclinical hypothyroidism and has been used in multiple previous studies.[26, 38, 50, 51] Some experts regard individuals with TSH above 10 mU/L as having overt hypothyroidism,[52] since this is usually an indication for thyroid replacement therapy. We performed a sensitivity analysis that showed similar results with both definitions of overt hypothyroidism (results not shown).

Conclusion

The guideline-recommended two-step approach assessing fT4 only in case of abnormal TSH may prevent measuring fT4 in as many as 93% of individuals with a very small risk of missing thyroid dysfunction. Our findings do not support the simultaneous initial measurement of both TSH and fT4.

Supporting information

S1 Fig. Sensitivity and specificity of different prediction score levels for classifying hypothyroidism.

https://doi.org/10.1371/journal.pone.0196631.s001

(TIF)

S2 Fig. Density of fT4 values among euthyroids.

The red lines indicate reference range limits of fT4 tests measured in this cohort (normal range 11.2–20.8 pmol/l). This graph shows fT4 among euthyroids is normally distributed, and the 169 participants whose TSH is normal but fT4 falls the reference range are predominantly healthy outliers.

https://doi.org/10.1371/journal.pone.0196631.s002

(TIF)

S1 Table. Prevalence of overt dysfunction within eu-, hypo-, and hyperthyroid groups.

https://doi.org/10.1371/journal.pone.0196631.s003

(PDF)

S2 Table. Univariate analysis of potential predictors of hypothyroidism.

https://doi.org/10.1371/journal.pone.0196631.s004

(PDF)

S3 Table. Univariate analysis of potential predictors of hyperthyroidism.

https://doi.org/10.1371/journal.pone.0196631.s005

(PDF)

S4 Table. Sensitivity and specificity at each score level.

https://doi.org/10.1371/journal.pone.0196631.s006

(PDF)

Acknowledgments

We thank Kali Tal for her editorial suggestions to a previous version of the manuscript.

References

  1. 1. Spencer CA, LoPresti JS, Patel A, Guttler RB, Eigen A, Shen D, et al. Applications of a new chemiluminometric thyrotropin assay to subnormal measurement. The Journal of clinical endocrinology and metabolism. 1990;70(2):453–60. pmid:2105333.
  2. 2. Cooper DS, Biondi B. Subclinical thyroid disease. Lancet. 2012;379(9821):1142–54. pmid:22273398.
  3. 3. Biondi B, Cooper DS. The clinical significance of subclinical thyroid dysfunction. Endocrine reviews. 2008;29(1):76–131. pmid:17991805.
  4. 4. Wartofsky L, Dickey RA. The evidence for a narrower thyrotropin reference range is compelling. The Journal of clinical endocrinology and metabolism. 2005;90(9):5483–8. pmid:16148345.
  5. 5. Brabant G, Beck-Peccoz P, Jarzab B, Laurberg P, Orgiazzi J, Szabolcs I, et al. Is there a need to redefine the upper normal limit of TSH? European journal of endocrinology / European Federation of Endocrine Societies. 2006;154(5):633–7. pmid:16645008.
  6. 6. Surks MI, Goswami G, Daniels GH. The thyrotropin reference range should remain unchanged. The Journal of clinical endocrinology and metabolism. 2005;90(9):5489–96. pmid:16148346.
  7. 7. Baloch Z, Carayon P, Conte-Devolx B, Demers LM, Feldt-Rasmussen U, Henry JF, et al. Laboratory medicine practice guidelines. Laboratory support for the diagnosis and monitoring of thyroid disease. Thyroid: official journal of the American Thyroid Association. 2003;13(1):3–126. pmid:12625976.
  8. 8. Surks MI, Ortiz E, Daniels GH, Sawin CT, Col NF, Cobin RH, et al. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. Jama. 2004;291(2):228–38. pmid:14722150.
  9. 9. Canaris GJ, Manowitz NR, Mayor G, Ridgway EC. The Colorado thyroid disease prevalence study. Archives of internal medicine. 2000;160(4):526–34. pmid:10695693.
  10. 10. Toubert ME, Chevret S, Cassinat B, Schlageter MH, Beressi JP, Rain JD. From guidelines to hospital practice: reducing inappropriate ordering of thyroid hormone and antibody tests. European journal of endocrinology / European Federation of Endocrine Societies. 2000;142(6):605–10. pmid:10822223.
  11. 11. Gibbons V, Lillis S, Conaglen JV, Lawrenson R. Do general practitioners use thyroid stimulating hormone assay for opportunistic screening? The New Zealand medical journal. 2009;122(1301):25–30. pmid:19829389.
  12. 12. LeFevre ML, Force USPST. Screening for thyroid dysfunction: U.S. Preventive Services Task Force recommendation statement. Annals of internal medicine. 2015;162(9):641–50. pmid:25798805.
  13. 13. Hadlow NC, Rothacker KM, Wardrop R, Brown SJ, Lim EM, Walsh JP. The relationship between TSH and free T(4) in a large population is complex and nonlinear and differs by age and sex. The Journal of clinical endocrinology and metabolism. 2013;98(7):2936–43. pmid:23671314.
  14. 14. Biondi B, Bartalena L, Cooper DS, Hegedus L, Laurberg P, Kahaly GJ. The 2015 European Thyroid Association Guidelines on Diagnosis and Treatment of Endogenous Subclinical Hyperthyroidism. Eur Thyroid J. 2015;4(3):149–63. pmid:26558232; PubMed Central PMCID: PMCPMC4637513.
  15. 15. Viera AJ. Thyroid function testing in outpatients: are both sensitive thyrotropin (sTSH) and free thyroxine (FT4) necessary? Family medicine. 2003;35(6):408–10. pmid:12817866.
  16. 16. Surks MI, Chopra IJ, Mariash CN, Nicoloff JT, Solomon DH. American Thyroid Association guidelines for use of laboratory tests in thyroid disorders. Jama. 1990;263(11):1529–32. pmid:2308185.
  17. 17. Ladenson PW, Singer PA, Ain KB, Bagchi N, Bigos ST, Levy EG, et al. American Thyroid Association guidelines for detection of thyroid dysfunction. Archives of internal medicine. 2000;160(11):1573–5. pmid:10847249.
  18. 18. Garber JR, Cobin RH, Gharib H, Hennessey JV, Klein I, Mechanick JI, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocrine practice: official journal of the American College of Endocrinology and the American Association of Clinical Endocrinologists. 2012;18(6):988–1028. pmid:23246686.
  19. 19. The Association for Clinical Biochemistry BT, Foundation AaBT. UK Guidelines for the Use of Thyroid Function Tests. http://www.british-thyroid-association.org/infoforpatients/Docs/TFT_guideline_final_version_July_2006.pdf. 2006.
  20. 20. Emerson JF, Emerson SS. The impact of requisition design on laboratory utilization. American journal of clinical pathology. 2001;116(6):879–84. pmid:11764077.
  21. 21. Services. TNAoViD. Reducing unwarrented variation to increase value and improve quality. RighCare. http://fingertipspheorguk/documents/Atlas_2013%20Diagnosticspdf. November 2013.
  22. 22. CostHelper I. costhelper-what people ar paying. http://health.costhelper.com/thyroid.html. 2016.
  23. 23. Bauer DC, Brown AN. Sensitive thyrotropin and free thyroxine testing in outpatients. Are both necessary? Archives of internal medicine. 1996;156(20):2333–7. pmid:8911240.
  24. 24. Kende M, Kandapu S. Evaluation of thyroid stimulating hormone (TSH) alone as a first-line thyroid function test (TFT) in Papua New Guinea. P N G Med J. 2002;45(3–4):197–9. pmid:12968789.
  25. 25. Blum MR, Bauer DC, Collet TH, Fink HA, Cappola AR, da Costa BR, et al. Subclinical thyroid dysfunction and fracture risk: a meta-analysis. Jama. 2015;313(20):2055–65. pmid:26010634; PubMed Central PMCID: PMC4729304.
  26. 26. Rodondi N, den Elzen WP, Bauer DC, Cappola AR, Razvi S, Walsh JP, et al. Subclinical hypothyroidism and the risk of coronary heart disease and mortality. Jama. 2010;304(12):1365–74. pmid:20858880; PubMed Central PMCID: PMC3923470.
  27. 27. O'Leary PC, Feddema PH, Michelangeli VP, Leedman PJ, Chew GT, Knuiman M, et al. Investigations of thyroid hormones and antibodies based on a community health survey: the Busselton thyroid study. Clinical endocrinology. 2006;64(1):97–104. pmid:16402936.
  28. 28. Helfand M, Force USPST. Screening for subclinical thyroid dysfunction in nonpregnant adults: a summary of the evidence for the U.S. Preventive Services Task Force. Annals of internal medicine. 2004;140(2):128–41. pmid:14734337.
  29. 29. Brown SJ, Bremner AP, Hadlow NC, Feddema P, Leedman PJ, O'Leary PC, et al. The log TSH-free T4 relationship in a community-based cohort is nonlinear and is influenced by age, smoking and thyroid peroxidase antibody status. Clinical endocrinology. 2016;85(5):789–96. pmid:27197788.
  30. 30. Manji N, Carr-Smith JD, Boelaert K, Allahabadia A, Armitage M, Chatterjee VK, et al. Influences of age, gender, smoking, and family history on autoimmune thyroid disease phenotype. The Journal of clinical endocrinology and metabolism. 2006;91(12):4873–80. pmid:16968788.
  31. 31. Knudsen N, Laurberg P, Rasmussen LB, Bulow I, Perrild H, Ovesen L, et al. Small differences in thyroid function may be important for body mass index and the occurrence of obesity in the population. The Journal of clinical endocrinology and metabolism. 2005;90(7):4019–24. pmid:15870128.
  32. 32. Jabbar A, Pingitore A, Pearce SH, Zaman A, Iervasi G, Razvi S. Thyroid hormones and cardiovascular disease. Nat Rev Cardiol. 2016. pmid:27811932.
  33. 33. Moons KG, Harrell FE, Steyerberg EW. Should scoring rules be based on odds ratios or regression coefficients? Journal of clinical epidemiology. 2002;55(10):1054–5. pmid:12464384.
  34. 34. Vaidya B, Ukoumunne OC, Shuttleworth J, Bromley A, Lewis A, Hyde C, et al. Variability in thyroid function test requests across general practices in south-west England. Quality in primary care. 2013;21(3):143–8. pmid:23968263.
  35. 35. Daucourt V, Saillour-Glenisson F, Michel P, Jutand MA, Abouelfath A. A multicenter cluster randomized controlled trial of strategies to improve thyroid function testing. Medical care. 2003;41(3):432–41. pmid:12618646.
  36. 36. Willis EA, Datta BN. Effect of an educational intervention on requesting behaviour by a medical admission unit. Annals of clinical biochemistry. 2013;50(Pt 2):166–8. pmid:23345588.
  37. 37. Tomlin A, Dovey S, Gauld R, Tilyard M. Better use of primary care laboratory services following interventions to 'market' clinical guidelines in New Zealand: a controlled before-and-after study. BMJ quality & safety. 2011;20(3):282–90. pmid:21228434.
  38. 38. Gencer B, Collet TH, Virgini V, Bauer DC, Gussekloo J, Cappola AR, et al. Subclinical thyroid dysfunction and the risk of heart failure events: an individual participant data analysis from 6 prospective cohorts. Circulation. 2012;126(9):1040–9. pmid:22821943; PubMed Central PMCID: PMCPMC3884576.
  39. 39. Pearce SH, Brabant G, Duntas LH, Monzani F, Peeters RP, Razvi S, et al. 2013 ETA Guideline: Management of Subclinical Hypothyroidism. Eur Thyroid J. 2013;2(4):215–28. pmid:24783053; PubMed Central PMCID: PMCPMC3923601.
  40. 40. Billewicz WZ, Chapman RS, Crooks J, Day ME, Gossage J, Wayne E, et al. Statistical methods applied to the diagnosis of hypothyroidism. Q J Med. 1969;38(150):255–66. pmid:4181088.
  41. 41. Carle A, Pedersen IB, Knudsen N, Perrild H, Ovesen L, Andersen S, et al. Hypothyroid Symptoms Fail to Predict Thyroid Insufficiency in Old People: A Population-Based Case-Control Study. The American journal of medicine. 2016;129(10):1082–92. pmid:27393881.
  42. 42. Murray IP. The Clinical Diagnosis of Thyroid Disease. Med J Aust. 1964;1:827–31. pmid:14166700.
  43. 43. Seshadri MS, Samuel BU, Kanagasabapathy AS, Cherian AM. Clinical scoring system for hypothyroidism: is it useful? J Gen Intern Med. 1989;4(6):490–2. pmid:2585156.
  44. 44. Chopra IJ. Clinical review 86: Euthyroid sick syndrome: is it a misnomer? The Journal of clinical endocrinology and metabolism. 1997;82(2):329–34. pmid:9024211.
  45. 45. Refetoff S, Bassett JH, Beck-Peccoz P, Bernal J, Brent G, Chatterjee K, et al. Classification and proposed nomenclature for inherited defects of thyroid hormone action, cell transport, and metabolism. The Journal of clinical endocrinology and metabolism. 2014;99(3):768–70. pmid:24823702; PubMed Central PMCID: PMCPMC3942236.
  46. 46. Gurnell M, Halsall DJ, Chatterjee VK. What should be done when thyroid function tests do not make sense? Clinical endocrinology. 2011;74(6):673–8. pmid:21521292.
  47. 47. Brucker-Davis F, Oldfield EH, Skarulis MC, Doppman JL, Weintraub BD. Thyrotropin-secreting pituitary tumors: diagnostic criteria, thyroid hormone sensitivity, and treatment outcome in 25 patients followed at the National Institutes of Health. The Journal of clinical endocrinology and metabolism. 1999;84(2):476–86. pmid:10022404.
  48. 48. Olateju TO, Vanderpump MP. Thyroid hormone resistance. Annals of clinical biochemistry. 2006;43(Pt 6):431–40. pmid:17132274.
  49. 49. Persani L. Clinical review: Central hypothyroidism: pathogenic, diagnostic, and therapeutic challenges. The Journal of clinical endocrinology and metabolism. 2012;97(9):3068–78. pmid:22851492.
  50. 50. Baumgartner C, da Costa BR, Collet TH, Feller M, Floriani C, Bauer DC, et al. Thyroid Function Within the Normal Range, Subclinical Hypothyroidism, and the Risk of Atrial Fibrillation. Circulation. 2017;136(22):2100–16. pmid:29061566; PubMed Central PMCID: PMCPMC5705446.
  51. 51. Chaker L, Baumgartner C, den Elzen WP, Ikram MA, Blum MR, Collet TH, et al. Subclinical Hypothyroidism and the Risk of Stroke Events and Fatal Stroke: An Individual Participant Data Analysis. The Journal of clinical endocrinology and metabolism. 2015;100(6):2181–91. pmid:25856213; PubMed Central PMCID: PMCPMC4454799.
  52. 52. Khandelwal D, Tandon N. Overt and subclinical hypothyroidism: who to treat and how. Drugs. 2012;72(1):17–33. pmid:22191793.