Funny thyroid function tests

Dr Mark Gurnell

Addenbrooke’s Hospital, Cambridge

Summer School 11-14 July 2006
The Møller Centre, Storeys Way, Cambridge, UK

Accurate diagnosis of thyroid disease is dependent on understanding hypothalamic-pituitary-thyroid physiology and the pathophysiology that can affect this classical endocrine feedback loop. Even minor perturbations of thyroid status, which may be imperceptible to the patient and clinician, can significantly alter the relationship between circulating thyroid hormone levels and pituitary thyrotropin (TSH), such is the finely-tuned nature of the axis.

Whilst thyroid disease in its most florid forms is easily recognised, minor disturbances of thyroid function can prove difficult to diagnose, whilst other endocrine and non-endocrine disorders can occasionally mimic hyper- or hypo- thyroidism. Therefore, in most cases establishing the correct diagnosis ultimately depends on the accurate interpretation of thyroid function tests.

Over the past two decades, there have been significant improvements in assays for free thyroxine (FT4), free triiodothyronine (FT3) and TSH, and in the majority of subjects with suspected thyroid dysfunction interpretation of thyroid function tests is straightforward. However, there remain a small, but important set of conditions in which the results of FT4, FT3 and TSH assays ‘fail to agree’, and when the ‘golden rules’ that govern the relationship between free hormone levels and thyrotropin appear to be, or are indeed, broken – cases with ‘funny thyroid function tests (TFTs)’ (Table 1).

Table 1 Patterns of ‘funny TFTs’ and associated conditions

In this talk I will use a series of clinical vignettes, describing subjects referred to our clinic with ‘anomalous’ thyroid function tests, to illustrate important pitfalls that await the unwary clinician who fails to acknowledge, and thus appropriately investigate, patients with ‘funny TFTs’.

References

1. Dayan CM, 2001. Interpretation of thyroid function tests. Lancet, 357:619-624
2. Gurnell M, Beck-Peccoz PB & Chatterjee VKK, 2005. Resistance to Thyroid Hormone. In: Endocrinology (5th Edition), Ed. LJ DeGroot & JL Jameson, Elsevier Saunders, Philidelphia, 2227-2237.

The opinions expressed in this paper are those of the speaker and do not necessarily reflect the views of the Society

Revised: 23-Aug-2006