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Thyroid function tests and hypothyroidism

BMJ 2003; 326 doi: (Published 08 February 2003) Cite this as: BMJ 2003;326:295

Measurement of serum TSH alone may not always reflect thyroid status

  1. Anthony D Toft (, consultant physician,
  2. Geoffrey J Beckett, reader
  1. Endocrine Clinic, Royal Infirmary, Edinburgh EH3 9YW
  2. University Department of Clinical Biochemistry, Royal Infirmary

    Paper p 311

    It is extraordinary that more than 100 years since the first description of the treatment of hypothyroidism and the current availability of refined diagnostic tests, debate is continuing about its diagnosis and management. Symptoms of thyroid failure are often non-specific, such as weight gain, low mood, and fatigue. Some patients seeking an explanation for feeling “below par” are disappointed when thyroid function tests are normal. Unable to accept that there may be psychosocial reasons for their symptoms, a vociferous minority believe that hypothyroidism may exist with normal serum concentrations of both thyroxine (T4) and thyroid stimulating hormone (TSH).

    Their hypothesis is that a doctor cannot know whether a concentration of free T4 or TSH within wide reference ranges is normal for that individual. Such an argument, supported by some misguided medical practitioners to justify prescribing various combinations of thyroid hormones, does not appreciate the sensitivity of the pituitary thyrotroph, which modifies the synthesis and secretion of TSH in response to minor changes in thyroid hormone concentrations within their reference ranges. For example, a reduction in free T4 from 20 pmol/l to 15 pmol/l is likely to cause …

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