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Biochemical assessment of adequate levothyroxine replacement in primary hypothyroidism differs with different TSH assays: potential clinical implications
  1. Tejas R Kalaria1,
  2. Anna Sanders2,
  3. Clare Ford1,
  4. Harit Buch3,
  5. Jonathan Samuel Fenn1,
  6. Helen L Ashby2,
  7. Pervaz Mohammed2,
  8. Rousseau Mariano Gama1,4
  1. 1 Clinical Biochemistry, Black Country Pathology Services, New Cross Hospital, Royal Wolverhampton Hospitals NHS Trust, Wolverhampton, UK
  2. 2 Clinical Biochemistry, Black Country Pathology Services, Russells Hall Hospital, Royal Wolverhampton Hospitals NHS Trust, Dudley, UK
  3. 3 Endocrinology and Diabetes, Royal Wolverhampton Hospitals NHS Trust, Wolverhampton, UK
  4. 4 School of Medicine and Clinical Practice, University of Wolverhampton, Wolverhampton, UK
  1. Correspondence to Dr Tejas R Kalaria, Clinical Biochemistry, Black Country Pathology Services, New Cross Hospital, Royal Wolverhampton Hospitals NHS Trust, Wolverhampton WV10 0QP, UK; tejaskumar.kalaria{at}nhs.net

Abstract

Aim Thyroid stimulating hormone (TSH) assays provided by Abbott Laboratories and Roche Diagnostics are used by approximately 75% of laboratories in the UK. We assessed the potential impact of Abbott and Roche TSH assay differences on the biochemical assessment of levothyroxine replacement in primary hypothyroidism.

Method Samples from 100 consecutive primary care patients (83 women, median age 64 years, IQR 51–73 years) with primary hypothyroidism on adequate levothyroxine based on an Abbott Architect TSH in the reference range were analysed for TSH on Roche cobas within 24 hours. The Abbott and Roche TSH results were compared. Over 1 year, TSH results from patients in primary care from the laboratories with Abbott and Roche methods were compared.

Results The median (IQR) Roche TSH (2.5 (1.3–3.6) mIU/L) was 30%±10% higher (p<0.001) than Abbott TSH (1.9 (1.1–2.6) mIU/L). Although all Abbott TSH results were in the Abbott specific reference range, 14 patients (14%) had Roche TSH results above the Roche specific reference range. In the 1 year gather, Roche TSH (1.9 (1.3–2.9) mIU/L, n=103 932) results were higher (p<0.001) than Abbott TSH (1.5 (1.0–2.2) mIU/L, n=1 10 544) results. The TSH results were above their assay-specific upper reference limit in 10.7% of Roche results and 4.2% of Abbott results.

Conclusion Biochemical assessment of levothyroxine replacement may be dependent on the type of TSH assay. Laboratorians and clinicians should be aware that the lack of harmonisation between TSH methods and their assay-specific reference ranges may potentially lead to different patient management decisions. We suggest lot verification in laboratories should include processes to identify cumulative drift in assay performance.

  • thyroid diseases
  • chemistry
  • clinical
  • endocrinology

Data availability statement

Data are available from the corresponding author on request.

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Background

Primary hypothyroidism is a common chronic endocrine disorder with just over 3% of the population in England on levothyroxine replacement.1 Treatment with levothyroxine is monitored by measuring serum thyroid stimulating hormone (TSH).2 3 The dose of levothyroxine is adjusted, if required, to achieve optimal well-being while achieving normalisation of TSH and avoiding overtreatment.2 3 Monitoring of TSH is also recommended to check compliance and adjust levothyroxine dose requirements caused by concomitant medication and increasing age.3 Some laboratories, including one of the two laboratories participating in this study, provide rule-based automated comments on optimising levothyroxine replacement based on thyroid function tests (TFTs) results.4

Method bias and variation in reference intervals between assays are well recognised, including those for different TSH assays.5 Between method TSH assay differences, however, are not considered in clinical guidelines on the management of primary hypothyroidism2 6 7 and therefore any impact in the clinical management of primary hypothyroidism is not widely appreciated.

TSH assays provided by Roche Diagnostics and Abbott Laboratories are used by about 75% of clinical laboratories in the UK including all five hospital laboratories in our regional pathology service. We, therefore, evaluated the impact of between method differences in the Roche and Abbott TSH assays on the biochemical assessment of levothyroxine replacement in primary hypothyroidism.

Patients and methods

In an institution approved and registered service evaluation project, samples from 100 consecutive patients in primary care with primary hypothyroidism on adequate levothyroxine based on an Abbott Architect TSH in the reference range were analysed for TSH on Roche cobas within 24 hours. Samples from patients who were pregnant or children (<18 years) were excluded. Samples were identified at the laboratory with an Abbott platform, as electronic requesting of TFTs at this site includes a drop-down box acquiring data whether ‘on thyroxine’. The diagnosis of primary hypothyroidism and repeat prescription of levothyroxine were confirmed from the electronic primary care medical records.

Additionally, TSH results from primary care samples from both laboratories during March 2019 to February 2020 were compared to further investigate the relationship between Abbott and Roche TSH assays. The samples were not from the same patients but since the laboratories provide pathology services for neighbouring districts in the same geographic area, it was assumed that the population and thyroid test requesting practice would be similar. Samples in both the laboratories were collected in similar serum separator clot activator tubes (454214, Greiner, Austria) and, being part of the same pathology network, both the laboratories had analogous sample transport and handling practices.

TSH was measured on an Abbott Architect i2000 SR (Abbott Laboratories, USA) and a Roche cobas e801 (Roche Diagnostics, GmbH, Mannheim, Germany). There were no TSH reagent lot changes in either laboratory during analyses of samples from patients with primary hypothyroidism and neither laboratory identified any lot-to-lot performance shifts during 1-year period when TSH results from primary care samples were studied. The inter-assay coefficient of variation for Abbott TSH was 6.8% at 3.4 mIU/L and 6.6% at 17.8 mIU/L and for Roche TSH was 2.0% at 3.4 mIU/L and 2.2% at 26.7 mIU/L. Both laboratories use manufacturer-provided assay-specific TSH reference ranges for interpretation of results; these were 0.35–4.94 mIU/L and 0.27–4.20 mIU/L for the Abbott and Roche assays, respectively. The laboratories are ISO 15189 accredited and both assays had satisfactory internal quality control and external quality assurance performance during the study periods.

Data were tabulated in Excel (Microsoft Corporation) and statistical analysis performed using IBM SPSS Statistics for Windows, V.25 (IBM Corp). Since the data were non-parametric (Shapiro-Wilk test), Spearman rank correlation was used to measure the degree of association between the Abbott and Roche results. Wilcoxon signed-rank and Mann-Whitney U tests were used to assess the significance of the difference between paired and unpaired Abbott and Roche TSH results, respectively. Data are expressed as medians with IQRs. The threshold for statistical significance was 5%.

Results

The median (IQR) age of the 100 patient cohort prescribed levothyroxine for primary hypothyroidism was 64 (51–73) years and 83% were female. Abbott and Roche TSH results correlated (r 0.988, p<0.001) (online supplemental figure 1). Roche TSH (2.5 (1.3–3.6) mIU/L) was 30%±10% higher (Wilcoxon signed-rank Z=8.65, p<0.001) than Abbott TSH (1.9 mIU/L (1.1–2.6) mIU/L) (online supplemental figure 2). All 100 patients had Abbott TSH values within the Abbott specific reference range; of these 86 had normal Roche TSH values and 14 had Roche TSH results above the Roche specific upper reference range (figure 1).

Supplemental material

Figure 1

Paired TSH results by Roche and Abbott assays for patients with primary hypothyroidism treated with levothyroxine (n=100). TSH, thyroid stimulating hormone; URL, upper reference range.

During March 2019 to February 2020, the laboratories with Abbott and Roche analytical platforms performed 110 544 (61% women, median age 58 years, IQR 44–72) and 103 932 (61% women, median age 61 years, IQR 46–74) TSH analyses, respectively (figure 2). Roche TSH results (1.9 (1.3–2.9) mIU/L) were 30% higher (Mann-Whitney test Z=89.8, p<0.001) than Abbott (1.5 (1.0–2.2) mIU/L). The 2.5th and 97.5th percentiles were 0.28 and 7.6 mIU/L for Roche TSH and 0.15 and 6.3 mIU/L for Abbott TSH results, respectively. Of these, 4.2% of the Abbott TSH and 10.7% of the Roche TSH results were above their assay-specific upper reference limits.

Figure 2

Distribution of serum TSH results over 1 year from primary care samples for Abbott (n=110 544) and Roche (n=103 932) analytical platforms. Displayed results are from limit of quantitation to TSH 7.0 mIU/L. TSH, thyroid stimulating hormone; URL, upper reference limit.

Discussion

Comparison of TSH results from patients on levothyroxine for primary hypothyroidism showed that Roche TSH results were on average 30% higher than the Abbott TSH results. This finding was reproduced in primary care patient TSH results generated by the Roche and Abbott methods over 1 year. Additionally, non-comparability of respective reference intervals with the prevailing assay performance may potentially contribute to differing management decisions.

In this study, 14% of patients on thyroxine replacement for primary hypothyroidism had discordant Abbott and Roche TSH results with potential clinical implications. In these patients, TSH measured with the Abbott assay were within the manufacturer provided reference limits and no further action would be required. If, however, TSH had been measured with the Roche assay it would have been elevated in 14% of patients raising potential issues around compliance, malabsorption and drug interactions, and then if appropriate levothyroxine dose adjustment in accordance with current management guidelines.2 3 6 8 It is, however, unclear which assay results more accurately reflect the clinical status and, therefore, it is possible that these patients monitored with the Abbott TSH assay were under-replaced with levothyroxine. Although not studied in this report, it is also possible that patients considered adequately replaced on the Roche TSH assay may have been considered over-replaced if TSH had been monitored using the Abbott assay.

The 1-year data gather on TSH in primary care samples showed that 10.7% of Roche TSH results were above their assay-specific upper reference limit compared with 4.2% Abbott TSH results. There may be several reasons for this, including differences in patient cohorts and clinical practice, but also suggests that the impact of assay discordance affecting patient management could be significant as approximately 75% of laboratories in the UK use either Roche or Abbott TSH assays and since over 3% of the population in England are prescribed levothyroxine replacement.1

The discordant TSH results are due to between method bias and variation in the manufacturer provided reference ranges. The Abbott TSH assay has a negative proportionate bias compared with the Roche TSH assay and despite this, the Abbott TSH upper reference limit is higher than that of the Roche TSH assay (figure 1). In addition, the method bias has recently become increasingly divergent with Abbott TSH and Roche TSH displaying downward and upward drifts, respectively (commentary: UK NEQAS for Thyroid Hormones, distribution 444). This, therefore, raises an issue regarding the appropriateness of manufacturer-provided reference ranges and especially related to current assay performance.

The manufacturers did not issue any field safety notices nor did the laboratories identify assay performance concerns while the assays were slowly drifting apart. This indicates a limitation of the traditional lot verification procedure employed by laboratories and manufacturers9 10 where results from each new lot are only compared against results from the existing lot. This approach could fail to identify small cumulative shifts in assay performance as change with each new lot may not be significantly different from the preceding lot but with subsequent lot changes the cumulative difference gradually becomes potentially clinically significant but remains unnoticed.11 This is especially important for analytes, like TSH, used for long term monitoring of chronic health conditions.9 Monitoring of the distributions of patient results over time,10 moving patient averages,10 comparison of regression equations from successive lot evaluations11 and external quality assurance data12 may be useful in identifying gradual drift. These results, especially the gradual drift since the last harmonisation attempt,13 also emphasise the critical importance of manufacturers in maintaining stringent lot verification and traceability of their calibration materials and methods.

This study also underscores the limitations of universal reference ranges for TSH14 and single TSH cut-offs from clinical guidelines3 since these do not take into account between assay differences. Immunoassay harmonisation is problematic and despite discussions regarding more sensitive and specific methods for thyroid hormone measurement like mass spectrometry,15 immunoassays are likely to remain the preferred analytical method as they are easily automated and have a low cost, high throughput and excellent precision. We, therefore, suggest the current challenge for laboratories and assay manufacturers is in optimising assay-specific reference ranges for prevailing assay performance since any derived assay-specific reference intervals13 16 may not be valid in the presence of significant assay drift. Manufacturer-provided reference intervals are valid at the time of derivation and the decision by laboratories to use them constitutes risk transfer. The continued use of reference intervals over time by laboratories without consideration of assay performance may further cascade the risk to the service users.

In summary, clinicians and laboratorians should be aware that the biochemical assessment of levothyroxine replacement in patients with primary hypothyroidism using different TSH assays may be strikingly different and based on clinical guidelines may lead to different clinical management decisions.

Take home messages

  • Between method differences are well recognised but generally considered to be accounted for by their different method-specific reference intervals. We provide data that this may be a false assumption, at least for thyroid stimulating hormone (TSH).

  • Biochemical assessment of levothyroxine replacement in patients with primary hypothyroidism using different TSH assays may be different and may lead to different clinical management decisions.

  • Lot verification in laboratories should include processes to identify cumulative drift in assay performance.

Data availability statement

Data are available from the corresponding author on request.

Ethics statements

Patient consent for publication

Ethics approval

Institution approved service evaluation project ID 5441 with the Royal Wolverhampton NHS Trust.

Acknowledgments

The authors thank Jordan Danks for coordinating sample exchange and helping with data entry.

References

Footnotes

  • Handling editor Tahir S Pillay.

  • Contributors AS and TRK designed the service evaluation under guidance of RMG. JSF, AS and TRK coordinated sample analysis and collected the laboratory and clinical data. TRK analysed data and wrote the first draft of the manuscript. All the authors contributed to data interpretation, critically reviewed the manuscript and approved the final version.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.