ABSTRACT
This chapter discusses the biosynthesis and transport of thyroid hormones and the regulation of thyroid function by the hypothalamic-pituitary complex will suffice. The central role of the thyroid gland in controlling metabolism was recognized in the 19th century, but evaluation of the function of the thyroid remains an evolving science. Radioimmunoassay depends upon measurement of the distribution of a tracer quantity of radiolabeled hormone that competes with the endogenous hormone in the patient’s specimen for binding to a capture antibody. The thyroid hormone binding ratio (THBR), another calculated value proportional to the fraction of hormone that is free in circulation, derives from measurement of the availability of protein-binding sites in the patient’s serum. Potential pitfalls in the interpretation of THBR tests occur when there is a ligand that can interfere with binding to both the solid phase and serum proteins, for example, nonthyroidal illness.
