Funny Thyroid Function Tests
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).