Standards for thyroid function testing strategies

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All Wales Clinical Biochemistry Audit

Group

Standards for Thyroid Function Testing

Strategies

INTRODUCTION

The results of a questionnaire sent to all

laboratories in Wales in September 1999 revealed significant differences in

thyroid function testing strategies between laboratories. The majority (9/14)

of laboratories used a first line thyrotropin (TSH) and free thyroxine (FT4)

strategy, 4 laboratories used TSH alone and 1 laboratory used FT4 alone. Some

of the strategies for further testing were unnecessarily complex.

Many guidelines for the detection of thyroid

disease recommend a first-line TSH-only strategy.1,2 However, a TSH-only

strategy may miss cases of secondary and tertiary hypothyroidism 3,4,5 as the

presenting clinical features of pituitary and hypothalamic disease are often

non-specific. A TSH-only strategy may also miss cases of primary

hyperthyroidism (G.Beckett, personal communication), as immunoassays remain

prone to interference, especially from heterophile antibodies.6,7

The survey showed that many laboratories

experienced problems with inappropriate testing (11/13) and excessive

repetition of testing (10/13). While method-related differences in reference

intervals were expected, even those quoted by laboratories using the same methodology

were different. It is often impractical for laboratories to validate reference

intervals on their own, but a multicentre approach for deriving them for

thyroid hormones using identical analysers has been proposed.8

The following standards are therefore

recommended in the light of the survey findings, discussion at audit meetings

in November 1999 and April 2000 and published evidence.

STANDARDS

1. All laboratories offering thyroid function

tests should provide TSH and FT4 assays “on site”

and should provide or have access to assays

for free tri-iodothyronine and thyroid antibodies.

TSH should be measured by at least a second

generation immunoassay with a functional

sensitivity down to 0.02 mU/L.

2. It is recommended that all laboratories

should use a TSH and FT4 first-line testing strategy.

3. Laboratories should have a clearly

documented strategy for further testing, which should be

as simple and explicit as possible. The

strategy should be agreed with local physicians and

endocrinologists. It is recommended that the

strategy should utilise only first-line test results,

previous results and clinical information.

4. It is recommended that measurement of

thyroid peroxidase (TPO) antibodies9 is generally

sufficient when antibody testing is required

to investigate suspected autoimmune thyroid

disease; thyroglobulin antibodies do not need

to be routinely measured. It is recommended

that TPO antibodies are measured using an

immunoassay technique; haemagglutination

methods have inadequate sensitivity and

specificity.

5. All laboratories should use validated

reference intervals. Laboratories using the same

method should endeavour to adopt common

reference intervals.

6. Laboratories should comment on abnormal

results where appropriate, especially for general practitioners and other

non-expert clinicians.

7. Laboratories should endeavour to

discourage inappropriate and unnecessary repeat testing.

8. Laboratories should be aware of potential

analytical interferences (e.g. heterophile antibodies) and either investigate

problems “on site” or refer samples to other laboratories for

checking.

9. Laboratories should ensure that

appropriate internal quality control (IQC) and external quality assessment

(EQA) procedures are in place for each assay they perform.

REFERENCES

1. Ladenson PW, Singer PA, Ain KB, et al.

American Thyroid Association Guidelines for the

detection of thyroid disease. Arch Intern Med

2000; 160: 1573-1575.

2. Bauer DC, Brown AN. Sensitive thyrotropin

and free thyroxine testing in outpatients. Are

both necessary? Arch Intern Med 1996; 156:

2333-2337.

3. Waise, A, Belchetz PE. Clinical review:

Lesson of the week. Unsuspected central

hypothyroidism. Testing only for thyroid

stimulating hormone will miss unsuspected cases of

central hypothyroidism. BMJ 2000; 321:

1275-1277.

4. Wardle CA, Fraser WD, Squire CR. Pitfalls

in the use of thyrotropin concentration as a firstline thyroid-function test.

Lancet 2001; 357: 1013-1014.

5. Dayan CM. Interpretation of thyroid

function tests. Lancet 2001; 357: 619-624.

6. Selby C. Interference in immunoassay. Ann

Clin Biochem 1999; 36: 704-721.

7. Ismail AAA, Barth JH. Wrong biochemistry

results. (editorial) BMJ 2001; 323: 705-706.

8. CE, Hanning I, McBain AM, Moody D,

Price A. A model for a multicentre approach to the derivation of reference

intervals for thyroid hormones and testosterone for laboratories

using identical analysers. Clin Chem Lab Med

2000; 38: 1013-1019.

8. tti S, Caturegli P, Piccolo P, et al.

Antithyroid peroxidase autoantibodies in thyroid

diseases. J Clin Endocrinol Metab 2000; 71:

661-669.

ACKNOWLEDGEMENTS

Mr.I.Hanning, Dr.I.Hainsworth and Dr.R..

VERSION: 1

DATE: 11 January 2005

Appendix: Calendar of Audit Process for

Standards for Thyroid Function Testing Strategies

Nov. 1999 Findings of a survey of Welsh

laboratories’ strategies for thyroid function testing

(14/14 laboratories replied) undertaken by

Dr.I.Hainsworth (Consultant Biochemist,

ton Hospital, Swansea) and presented at

a joint Wales/South West England

Clinical Biochemistry Audit meeting in

Weston-super-Mare on 18th November 1999.

2000-2001 Initial draft standards prepared by

Dr.I.Hainsworth and then modified by Mr.I.Hanning (Consultant Biochemist, Hull

Royal Infirmary) following Dr.Hainsworth’s retirement.

Additional advice provided by Dr.R.

(Consultant Biochemist, University Hospital

of Wales, Cardiff). Draft standards reviewed

at All Wales Clinical Biochemistry Audit

Group committee meetings.

2001 Draft standards sent for consultation to

consultant biochemists and endocrinologists

in Wales to seek their views. Final draft of

standards presented at the All Wales

Clinical Welsh Biochemistry Audit Group

meeting on 18th October 2001 and at the

Welsh Endocrine and Diabetes Society business

meeting on 1st November 2001.

Nov. 2001 Standards finalised and ratified at

an All Wales Clinical Biochemistry Audit Group

committee meeting (on 22nd November 2001) by

Dr.K.Griffiths (chairman).

2005

Proposed date of re-audit and review of standards

http://www.acbwales.org.uk/audit/guidelines/tft_stds.pdf