Tests must still be done in possible thyroid dysfunction

EDITOR—The article by O'Reilly on reassessment of thyroid function tests raises some important questions but is misleading in several respects.1 Clinical features must of course be given full consideration in the assessment of possible thyroid dysfunction, but appropriate tests must still be done.

The symptoms of both hyperthyroidism and hypothyroidism are non-specific and can be mimicked by other conditions. Thus the practice of prescribing thyroid treatment on a clinical basis alone without biochemical confirmation carries potential risks. The statement that “the clinical features of hypothyroidism … have been relegated to the status of historical curiosities” is absurd. What the doctor aims to do is not simply to categorise a patient into hypothyroidism, hyperthyroidism, or the subclinical variants but rather to make a full diagnostic assessment, of which thyroid function tests are one important facet. Surprisingly, O'Reilly makes no mention of autoantibody tests, which are also helpful in assessing thyroid disease.

With regard to hyperthyroidism, a reduced thyroid stimulating hormone concentration is not in fact diagnostic. Clinical assessment is imperative, and before thyrotoxicosis is diagnosed the thyroxine (and in some cases triiodothyronine) concentration should be checked. The practice of using results of thyroid stimulating hormone tests alone to indicate hyperthyroidism is to be deplored and has led to a mistaken diagnosis in several cases subsequently shown to be cases of hypopituitarism.

O'Reilly mentions the use of thyroid stimulating hormone for screening purposes; the figures quoted for misleading results in the general population are interesting but date from 10 or more years ago. Thyroid stimulating hormone assays have considerably improved since then, and thus these numbers may not now be relevant.

O'Reilly is probably correct in claiming that too many indiscriminate requests for thyroid stimulating hormone tests are made. In some situations, however, notably in pregnancy, thyroid tests are not performed …