Screening for central hypothyroidism is unjustified

Editor—If we do enough tests on enough people we will always pick up abnormalities; we must also take account of the false positive rate of any testing strategy. These points are missing from Waise and Belchetz's article on unsuspected central hypothyroidism.¹

Most patients in general practice for whom measurement of thyroid stimulating hormone concentration is requested have vague symptoms. This has a firm foundation: the incidence of primary subclinical hypothyroidism is high.² It seems inappropriate to apply a further test—to look for central hypothyroidism—to this group. All the cases reported by Waise and Belchetz had clinical evidence suggestive of the diagnosis, and this suggests that attention should be directed to clinical investigation, not additional testing.

Another pitfall of the strategy described for detecting central hypothyroidism is the analytical requirements. Assays of free thyroxine are invariably affected by concurrent illness.³,⁴ If laboratories were to assay free thyroxine and thyroid stimulating hormone concentrations in all samples received for untreated patients from general practice the picture of central hypothyroidism—a low free thyroxine and a normal thyroid stimulating hormone concentration—would be found in many because of concurrent systemic illness. This in turn would lead to expensive and unnecessary follow up.

What is needed is a randomised controlled trial comparing a group in which both free thyroxine and thyroid stimulating hormone concentrations are measured with one in which thyroid stimulating hormone concentration alone is measured. The false positive rate, clinical outcome, and costs (including subsequent visits to a consultant) could then be determined for the patients with central hypothyroidism who are detected.

When such data are available it may be possible to argue that there is a sufficient cost benefit to justify the increased central funding that would be required for all laboratories to offer a front line testing strategy of measurement of free thyroxine and thyroid stimulating hormone concentrations. Until then we must ask, what other disease with an incidence of <50 cases per million population do we screen for?