Clinical Chemistry of Thyroid Function Testing

Abstract

The chapter focusses on the application of various thyroid function tests and the underlying pathophysiology. The functional cells of the thyroid gland are arranged in follicles around a lumen containing colloid material rich in thyroglobulin (Tg). The thyroid hormones are transported in the plasma almost entirely bound to particular plasma proteins, which are referred to as thyroid hormone-binding proteins (TBPs). In humans, the TBPs include T4-binding globulin (TBG), T4-binding prealbumin (TBPA), and the nonspecific binder, albumin. The biosynthetic phases are trapping of iodide, conversion to an activated form, iodination of tyrosine within Tg, and coupling of the resultant monoiodotyrosine and diiodotyrosine to form T3 and T4. A subtle increase in thyroid function is sufficient to slightly diminish the thyroid stimulating hormone (TSH) response to thyotropin-releasing hormone and occurs in early pregnancy, although normal pregnant individuals remain euthyroid. Studies performed in normal pregnancy demonstrated that albumin and TBPA have altered affinities for T4 and suggest that the T4 that is attached to TBPA and albumin is taken up by tissues. The chapter discusses the effects of drugs on binding with other drug-induced effects on thyroid hormone binding. The effects of markedly increased T4 binding by drugs, resulting in euthyroid hyperthyroxinemia are also presented in the chapter.