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Thyroid function tests and hypothyroidism: Authors' reply

BMJ 2003; 326 doi: (Published 15 May 2003) Cite this as: BMJ 2003;326:1087

Rapid Response:

Should we not look to physiology?

EDITOR – I noted the letters (1, 2) in response to the Toft and
Beckett (3) with interest. Both letters (1, 2) raised concerns about over
treatment with thyroxine and the editorial recommendation to restore the
serum thyroid stimulating hormone to the low normal range (3). Would it
surprise the authors of the letters that the vast majority of normal
euthyroid subjects have a serum thyroid stimulating hormone between 0.3
and 2.0 mU/L when using an assay with a reference range of 0.5 – 3.5 mU/L?

This is because serum thyroid stimulating hormone does not have a Gaussian
distribution but has a skewed distribution with a median value of 1.35
mU/L and needs logarithmic transformation to give it a Gaussian
distribution. Accordingly, what Toft and Beckett (3) have advocated is in
keeping with normal physiological principles.

1 Crilly M. Reducing concentrations further would be harmful. BMJ
2003:326:1086. (17 May.)

2 Vanderpump MP, Franklyn JA. Restoring serum TSH to reference range
should be goal of replacement. BMJ 2003:326:1086-7. (17 May.)

3 Toft AD, Beckett GH. Thyroid function tests and hypothroidism. BMJ
2003;326:295-6. (8 February.)

Competing interests:  
I work in the same department as GH Beckett and have worked with AD Toft.

Competing interests: No competing interests

19 May 2003
Patrick J Twomey
Specialist Registrar
Department of Clinical Biochemistry, Royal Infirmary, Edinburgh EH16 4SA