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

Numerous disorders result in elevations of circulating T₄ levels without true thyrotoxicosis. These can be grouped into four large categories, including abnormalities in the concentration or function of thyroid hormone–binding proteins, the effects of pharmacologic agents, syndromes of generalized or peripheral thyroid hormone resistance, and variants of NTI. These conditions are summarized in Table 36-3. Among patients with disorders involving thyroid hormone– binding proteins, those with familial dysalbuminemic hyper-thyroxinemia and serum thyroxine-binding prealbumin elevations have a falsely elevated serum FT₄ index resulting from normal binding of the radiolabeled T₃ that is used in most thyroid hormone–binding resin or T₃-resin uptake assays. In addition, patients with familial dysalbuminemic hyperthyroxinemia,74a as well as persons with T₄ autoantibodies, may have falsely elevated serum FT₄ values, when determined using some FT₄ methodologies. The elevation in serum T₄ levels that is seen after the use of oral contrast agents and amiodarone in healthy persons
must be distinguished from the true hyperthyroidism that may occur through jodbasedow mechanisms in the subset of patients who are susceptible to this phenomenon (see Chap. 37).