Thyroid testing 10 years on

Comment: In his timely review of changing patterns of thyroid function testing, Davey suggests that Australian Health Insurance Commission (HIC) policy is responsible for the increased emphasis on a “TSH-first” strategy, with consequent containment of costs for other thyroid function tests. While this may in part be true, the trend towards initial TSH testing has been advocated worldwide1 following the development of TSH assays sufficiently sensitive to distinguish the typical suppressed TSH levels of thyrotoxicosis from normal levels. The developments documented by Davey are a consequence of technological development, perhaps enhanced by selective rebating as a result of HIC policy.

It is unfortunate that current HIC policy is sometimes described as prohibiting more complete thyroid function testing, unless TSH level is abnormal. Rebate policy does not prohibit any line of testing and it is because the “TSH-first” approach has some serious, well-documented deficiencies.2 Measurement of levels of thyroid hormone in addition to TSH is clearly sanctioned in HIC regulations when TSH level alone can be misleading, for example in suspected pituitary dysfunction, or in monitoring the treatment of thyroid dysfunction. The adverse consequences, both human and financial, of relying on TSH measurement alone in such situations can be serious and may outweigh the savings achieved by restrictive testing. It must be noted again that a normal concentration of immunoreactive TSH has no predictive value in ruling out potentially life-threatening hypopituitarism,3 which may present with prominent hypothyroid features.

The effective integration of clinical and laboratory investigation of potential thyroid dysfunction requires an active laboratory–clinical interface. There are over a dozen patterns of thyroid function — some trivial or inconvenient, some quite serious — that can be misdiagnosed or incorrectly managed if communication across this interface is inadequate.4 Effective communication requires relevant information from the clinician and a response to this information within the laboratory. It is a reality that current patterns of investigation in Australia frequently fall short of this ideal. If, as a result of automation and effective competition, the unit cost of assays can eventually be reduced in relation to the total cost of medical care, it may become appropriate to revert to a more complete panel of initial testing that integrates tropic hormone and target gland secretion, a strategy that remains the cornerstone of definitive endocrine investigation.