Radioimmunoassay of Thyroid Hormones

Abstract

The main role of the thyroid gland is to metabolize and to incorporate iodine into a variety of organic compounds, which include the metabolically active thyroid hormones, 3,5,3′,5′-tetraiodothyronine or thyroxine (T₄) and 3,5,3′-triiodothyronine (T₃), and several other precursors and degradation products, as summarized in Table 1 (for review see DeGroot et al. 1984; Ingbar 1985). T₄ is the most important secretory product of the gland; T₃ is also secreted by the thyroid, although it is mostly (about 80% of the daily production) formed by peripheral outer ring deiodination of T₄. Both T₄ and T₃ are synthesized within a large protein, called thyroglobulin, representing the main constituent of the colloid stored in the follicular lumen. Proteolytic degradation of thyroglobulin releases T₄ and T₃ into the bloodstream, while only very small amounts of thyroglobulin enter the circulation. Minute quantities of diiodotyrosine (DIT) and monoiodotyrosine (MIT) are also secreted by the thyroid, whereas all other iodinated compounds circulating in the serum are almost exclusively produced by peripheral stepwise degradation of T₄ (Fig.1).