In 1999, 890 000 measurements of thyroid stimulating hormone were performed by Scottish hospital laboratories—approximately one test for every six of Scotland's 5.1 million people.1 This number does not include tests performed in the non-NHS laboratories or as part of the screening programme for congenital hypothyroidism. Although laboratory statistics are not collected nationally in England and Wales, the market in the United Kingdom (population 59 million) for thyroid stimulating hormone diagnostic tests is currently estimated at 9-10 million each year.

A remarkable downgrading of the clinical aspects of hypothyroidism and hyperthyroidism has paralleled the inexorable increase in the number of thyroid function tests performed over the past 20 years. This has led to chaos in the diagnosis of hypothyroidism. It has been stated that a diagnosis of clinical hypothyroidism can be made on the basis of biochemical measurements alone and that signs and symptoms are unnecessary.2 Other authors protest, and maintain that biochemical tests can be misleading and that the diagnosis can be made on clinical grounds alone.3 In hyperthyroidism, a suppressed thyroid stimulating hormone concentration is currently the cornerstone of biochemical diagnosis. No numerical value has been assigned to the serum concentration of thyroid stimulating hormone below which suppression is considered to occur. This value varies from centre to centre depending on the sensitivity of the local assay. Thus, to many non-specialists the diagnosis of hyperthyroidism is also confusing.