Part of “CHAPTER 36 – ABNORMAL THYROID FUNCTION TEST RESULTS IN EUTHYROID PERSONS“

The frequency with which thyroid disease is encountered in the general population, the ease with which it is excluded in otherwise healthy persons, and the poor specificity of the individual signs and symptoms of thyroid dysfunction all have contributed to the practice of screening for thyroid disease in patients with nonthyroidal illness (NTI), in the search for a readily reversible component to the presenting illness. The resultant recognition of multiple abnormalities in parameters of thyroid function in patients who ultimately are deemed to be euthyroid has engendered the term euthyroid sick syndrome to describe these persons.1,2 Although early cross-sectional studies in such patients had revealed a baffling collection of low, normal, or high values for total and free thyroid hormones, subsequent work has suggested a continuum of change, within which a given patient’s status is determined by the severity and duration of illness as well as the presence of mitigating influences that are associated with the specific underlying disorder. Hence, an early and dramatic reduction in serum total and free triiodothyronine (T₃) levels occurs, followed, in cases of sufficient severity, by a depression in serum total thyroxine (T₄) and variable changes in serum free thyroxine (FT₄) levels. Concurrent with decrements in levels of circulating thyroid hormones, a seemingly inept response is seen at the hypothalamic-pituitary level; this persists until the recovery phase of the underlying illness, at which time a rise in thyroid-stimulating hormone (TSH) to even supranormal levels may be seen that coincides with a rapid recovery in T₄ and T₃ levels3 (Fig. 36-1).