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 Table of Contents  
ORIGINAL ARTICLE
Year : 2013  |  Volume : 10  |  Issue : 3  |  Page : 100-103

Evaluation of upper reference limit of TSH in adults: Review of biological upper reference limit for TSH on a "chemiluminescence based" analysis technique


1 Department of Biochemistry, St John's Medical College and Hospital, Koramangala, Bangalore, India
2 Department of Endocrinology, St John's Medical College and Hospital, Koramangala, Bangalore, India
3 Scientific Affairs, Beckman Coulter, Inc., Brea, USA

Date of Web Publication6-Aug-2013

Correspondence Address:
Sultana Furruqh
Department of Biochemistry, St. John's Medical College and Hospital, Sarjapur Road, Koramangala, Bangalore
India
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Source of Support: Statistical work of the final data, reagents & consumables provided by Beckman Coulter India Private Limited. Technical expertise and laboratory work provided by the Departments of Clinical Biochemistry & Endocrinology St. Johnís Medical College Hospital, Conflict of Interest: None


DOI: 10.4103/0973-0354.116137

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  Abstract 

Background: There was an identified need to review the use of the manufacturer provided upper reference limit (URL) of the thyroid stimulating hormone (TSH) assay for the Indian population. URL's provided by manufacturers are usually based on western populations. This study triggered from our clinical feedback, identified that there is indeed difference for the URL for the Indian population compared to what the manufacturer has provided for the TSH assay. Materials and Method: Prospective study with volunteers from St. John's Medial College Hospital general health check who were pre-screened and selected based on inclusion/exclusion criteria decided for the purpose. We utilised the Access ® range of Chemiluminescence based Immuno assay system from Beckman Coulter Inc., Brea. CA, USA for this study. As indicated in the "background", we investigated URL of the TSH assay. Results and Conclusion: Results strongly favored the reduction of the URL to be more suitable for the population covered in this study.

Keywords: Chemiluminescence technology, thyroid assays, thyroid stimulating hormone, upper reference limit


How to cite this article:
Furruqh S, Menezes G, Ganapathy B, Ayyar V, Dhruvakumar SH, Vaseem M, Seshan S. Evaluation of upper reference limit of TSH in adults: Review of biological upper reference limit for TSH on a "chemiluminescence based" analysis technique. Thyroid Res Pract 2013;10:100-3

How to cite this URL:
Furruqh S, Menezes G, Ganapathy B, Ayyar V, Dhruvakumar SH, Vaseem M, Seshan S. Evaluation of upper reference limit of TSH in adults: Review of biological upper reference limit for TSH on a "chemiluminescence based" analysis technique. Thyroid Res Pract [serial online] 2013 [cited 2019 Jul 15];10:100-3. Available from: http://www.thetrp.net/text.asp?2013/10/3/100/116137


  Introduction Top


Thyroid disease is one of the common endocrine disorders and measurement of thyroid stimulating hormone (TSH) is widely used in the screening, diagnosis, treatment and monitoring of the disease. With the advent of increasingly sensitive TSH tests (such as the one described in this manuscript), the lower limit of detection has been reduced significantly. Typically third generation assays for TSH have a functional sensitivity of 0.01 - 0.02 μIU/mL 5 . This has led to the use of TSH as the frontline test in the diagnosis of thyroid disease since sensitive assays can be reliably used to diagnose hyperthyroidism as well as hypothyroidism.

There has been considerable debate world-wide on the impact caused by the use of such sensitive assays on the reference intervals for the TSH assay. [1],[2],[3],[4],[5] "Subclinical" thyroid disease is attracting interest of late, leading to the question whether people with TSH within current reference intervals would benefit from treatment. There is increasing evidence that such individuals progress to overt thyroid disease. Also, the association of subclinical hyperthyroidism with cardiovascular and neuromuscular dysfunction has been reported. [6] Vanderpump MP et al,[7] demonstrated that patients with a TSH value above 2.0 μIU/mL who are positive for TPO antibodies develop overt hypothyroidism faster than those with levels below 2.0 μIU/mL. As such, it is evident that these people are likely to benefit from early identification and appropriate management.There has been considerable debate regarding the appropriate upper reference limit (URL) for TSH assays. Recently, the National Academy of Clinical Biochemistry (NACB) has proposed lowering this to 2.5 μIU/mL.

Clinicians from St. John's Medical College Hospital Bangalore, India have been querying in particular the upper reference limit of the TSH assay. The clinicians expressed a view that we should examine the upper reference limit of the assay that was employed by the laboratory. The assay under discussion was the Access TSH assay on the Access Immunoassay system from Beckman Coulter, Inc., Brea, CA. The upper reference limit [Table 1] provided by the manufacturer of the Access TSH assay has been slightly more than double, compared to that proposed by the NACB. The view was taken up seriously since TSH is used as the first line of screening prior to subjecting the individual to further laboratory work up.
Table 1: Referene intervals as per the manufacturer's IFU

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Given this background and the fact that there is no reference range data for the Access TSH assay that could be directly applied to the Indian population, we decided to investigate further and evaluate the "URL" in particular. According to the clinicians' feedback and literature published across the world it was evident that an investigation of the URL could be immediately beneficial to a larger section of the Indian population. Since the question centered on URL for an Indian population, we chose volunteer Indian men and women maintaining an even gender representation in the study.


  Materials and Methods Top


Samples

After due ethical clearances and consent from the volunteers, 500 apparently healthy Indian men and women from the general health checkup of St. John's Medical College Hospital, Bangalore, India were chosen based on the "Exclusion" criteria. Exclusion criteria include: subjects who have history of thyroid disease currently or in the past, subjects with a family history of thyroid disease (1st and/2nd degree relatives), subjects currently suffering from any illness, subjects with a history of smoking, and pregnant women. Subjects falling under any of the exclusion criteria were excluded from this study. As per clinical evaluation, volunteers were confirmed to be not suffering from any illness currently. Prior to the sample collection, subjects and their first and second degree relatives' thyroid history were collected. All samples were drawn between 8 and 10a.m. Samples were included after due sample integrity checks were performed. Samples that were icteric, lipemic or hemolytic were excluded from this study. The sample split consisted of 268 female and 232 male samples. These samples were analyzed within 8hours from the collection time after complete centrifugation and serum separation. Sample collections and analysis were performed between February and June, 2009.

Analysis

All thyroid assays were performed on a chemiluminescence-based analyzer (UniCelDxI 600 from Beckman Coulter, Inc., Brea, CA) in St. John's Medical College Hospital laboratory, Bangalore, India. The assays used were the Immuno Assay products from Beckman Coulter Inc., Brea, CA [Access Hypersensitive hTSH, Access Anti - TPO, Access T3, Access T4, Access fT4and Access test specific calibrators]. Quality Control serum from Bio-Rad, Hercules, CA [Lyphocheck - Immunoassay Plus] was used to assess performance quality.

Singlet measurements were performed in batches for TSH, Total T3, Total T4, fT4 and anti-TPO on all selected samples.

Method and Assay characteristics

All the Access assays used in this study are immunoenzymatic assays based on the Chemiluminescence technology. While the TSH and Anti TPO assays are based on "Sandwich" immunoassay principle, the Free-T4, Total T4 and total T3 assays are based on the Principle of "Competitive" immunoassay.

Assay characteristics are reported below as found in the product literatures provided by Beckman Coulter Inc. Brea CA .

The Access Hypersensitive hTSH assay used in this testing offers virtually no cross reactivity to other peptide hormones. The sensitivity and specificity of this test enables better discrimination between hyperthyroid and euthyroid patients. The format of the assay used in our testing offers a functional sensitivity of 0.01-0.02 μIU/mL consistent to the definition of a "Third Generation" functional sensitivity of the TSH assay. The Access Anti TPO assay offers a cut-off of <9 IU/mL for healthy thyroids (as per Beckman Coulter Inc.'s product literature with TSH values between 0.5 and 2 μIU/mL no goiter, no personal or family history of thyroid disease, and absence of non-thyroid autoimmune disease). The Access fT4 assay reports approximately between 0.25ng/dL and 6.0ng/dL (determined by lot-specific calibrator values). The Access Total T4 assay reports approximately between 0.50 and 30.0μg/dL (determined by lot-specific calibrator values). The Access Total T3 assay reports approximately between 0.1 and 8.0ng/mL (determined by lot-specific calibrator values).


  Results and Discussion Top


Out of the 500 pre-screened volunteer samples, we selected results from "Biochemically Normal" thyroids for the final calculations. We defined the "Biochemically Normal Thyroid"as one whose results for Total T3, Total T4, free T4 and anti TPO assays are within the current reference intervals as provided by the manufacturers - Beckman Coulter Inc. Brea. CA [Table 1]. Based on this selection criterion we had to leave out results from 181 volunteers since at least one among the results of fT4, Total T4, Total T3 or Anti TPO was outside such a reference range. Since TSH is the parameter whose URL is under investigation, we did not include TSH resultsas a criterion while arriving at "Biochemically Normal Thyroids". That left us with 319 usable TSH results for our calculation [(161 female and 158 male in the age group 14 to 60 years) [Table 2].
Table 2: Age range for the qualified 319 samples

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Statistical calculations on these 319 results were performed using the "Medcalc" statistical software from MedCalc Software, Broekstraat 52, 9030 Mariakerke, Belgium.

Presented above is the [Table 3] with information on the 181 samples whose results were discarded from the final calculations. It presents the number of results that were found to be out of the "test specific Reference intervals" provided by the manufacturer as explained earlier.
Table 3: Split & stats of discarded results

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Data from [Table 3] becomes significant when we remind ourselves that all the tests reported are not performed unless TSH showed any abnormality since that is the first line screening test and hence, decides whether any other test would be performed or not. All the discarded results pertain only to 181 samples since some samples had more than one assay result falling outside of the reference intervals provided by the manufacturer of the assays. Evidently, more number of samples (nearly 50%) had elevated Anti-TPO results among the 181 samples whose results had to be discarded.

The Histogram above in [Figure 1] presents the volunteers grouped into age brackets. As is seen, spread of age for the subjects whose results qualified to be included in the final calculations ranges from 14 years to 60 years with a higher density between 20 years and 39 years.
Figure 1: TSH age histogram

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Left shift of the median and the evidenced higher density of results lying more to the left in the box plot [Figure 2] for TSH for the selected 319 samples strongly suggests the lowering of the Upper Reference Limit.
Figure 2: TSH box plot

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Finally, the "Box plot" obtained from the Medcalc statistical software for the TSH results [Figure 2] was used to understand specific shifts (or skews) and distribution of the results.

This is further visible when we take a look at the summary statistics. Specific percentile, median and mean values were identified using the "Summary statistics" [Figure 3].
Figure 3: Summary Stats - TSH Normal Results

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Median value obtained from biochemically normal thyroids stands at 1.67 μIU/mL and the arithmetic mean at 1.89 μIU/ml. Both of them are close to each other indicating less dispersion of values of TSH in biochemically normal thyroids.

Compared to the manufacturer provided Upper Reference Limit of 5.6 μIU/ml, the 95th percentile reported in the summary statistics is lower and stands at 4.13 μIU/ml. Since our work involved examining the Upper Reference Limit (URL) of the TSH assay, we chose to stick to the 95th percentile itself instead of looking at the "Central 95th percentile".

Out of the rejected 181 results that were from biochemically abnormal thyroids, there were 22 samples that were within the current reference interval of TSH (especially between 4.13 and 5.60 μIU/mL). This constitutes a significant 12% of the population in the study who would benefit from the TSH assay if the URL is set at 4.13 μIU/mL. As discussed earlier since TSH is used as the first line of screening prior to deciding further laboratory work up, this percentage of beneficiaries is significant and could not be ignored.


  Conclusion Top


The clearly evident left shift in the box plot supported by the lowered absolute value of the 95 th percentile to 4.13 for the rigorously selected 319 samples, strongly indicate that there is need to lower the upper reference limit of the Access TSH assay based on the chemiluminescence technology offered by Beckman Coulter, Inc., Brea, CA from its current value of 5.60 μIU/ml to 4.13μIU/ml for the Indian population.

 
  References Top

1.Laboratory support for the diagnosis of Thyroid disease - NACB Archived volume 13/2002.  Back to cited text no. 1
    
2.Surks MI,Goswami G, Daniels GH.Controversy in clinical endocrinology: The thyrotropin reference range should remain unchanged. J Clin Endocrinol Metab 2005;90:5489-96.  Back to cited text no. 2
    
3.Surks MI, Ortiz E, Daniel GH, Sawin CT, Col NF, Cobin RH, et al. Subclinical thyroid disease; Scientific review and guidelines for diagnosis and management JAMA 2004;291:228-38.  Back to cited text no. 3
    
4.Beckett G, Mackenzie F. Thyroid guidelines - are TSH assays fit for the purpose? Ann Clin Biochem 2007;44:203-8.  Back to cited text no. 4
    
5.Baskin HJ, Cobin RH, Duick DS, Gharib H, Guttler RB, Kaplan MM, et al. American Association of Clinical Endocrinologists medical guidelines for clinical practice for the evaluation and treatment of hyperthyroidism and hypothyroidism. Endocr Pract 2002;8:457-9.  Back to cited text no. 5
    
6.Biondi B, Cooper DS. The clinical significance of subclinical thyroid dysfunction. Endocr Rev 2008;29:76-131.  Back to cited text no. 6
    
7.Vanderpump MP, Turnbridge WM, French JM, Appleton D, Clark F, Gremley E, et al. The incidence of thyroid disease in the community: A twenty year follow-up of the Whickham Survey. Clin Endocrinol (Oxf) 1995;43:55-68.  Back to cited text no. 7
    


    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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