The nature, determination, and clinical importance of blood iodine: a review.

In normal conditions, serum (or plasma) iodine is largely composed of thyroxine and iodide with small quantities of thyroactive iodothyronines, such as 3: 5: 3'-tri-iodothyronine and 3: 3'-di-iodothyronine (Gross and Pitt-Rivers, 1952; Roche, Lissitzky, and Michel, 1952a and b; Roche, Michel, Nunez, and Wolf, 1956). Another relatively less active compound, 3: 3': 5'-tri-iodothyronine, is present in thyroid gland hydrolysates (Roche et al., 1955) and may be present in plasma in quantities too small to detect, since it is not broken down by the thyroid (Roche et al., 1956). Monoand di-iodotyrosine, known to be present in thyroid hydrolysates (Harington and Randall, 1929; Fink, Dent, and Fink, 1947), are not found in normal sera, being deiodinated by an enzyme present in the thyroid gland (Roche, Michel, Michel, and Lissitzky, 1952). In certain types of familial goitre, however, monoand di-iodotyrosine are present in serum as a result of the absence of the thyroid deiodinating enzyme (Stanbury, Kassenaar, Meijer, and Terpstra, 1955; Querido, Stanbury, Kassenaar, and Meijer, 1956; Stanbury, Meijer, and Kassenaar, 1956). Thyroxine and, to a lesser extent, 3:5: 3'-triiodothyronine have been shown to be bound to a specific plasma protein, intermediate in properties between al and a2 globulins (Trevorrow, 1939; Taurog and Chaikoff, 1948; Gordon, Gross, O'Connor, and Pitt-Rivers, 1952; Deiss, Albright, and Larson, 1953; Petermann, Robbins, and Hamilton, 1954; Robbins, Petermann, and Rall, 1955). No data are available for mono-iodotyrosine or for other iodothyronines. The iodine fraction, which is precipitated with protein, mainly consists of thyroid hormone and is termed " protein-bound iodine (P.B.I.)." Red cells normally contain iodide but no iodinated amino-acids (Joliot, Courrier, Horeau, and