Iodine association with plasma protein

TRANSPORT OF THYROID HORMONES IN THE BLOOD Iodine circulates as both organic iodine (95%) and inorganic iodide (5%). Most organic iodine is in T (90-95%), while Tj contains approximately 5%. Both T and T are transported in the blood in strong but noncovalent association with plasma proteins. 

As shown in Table IVA, the cells bind approximately 5% of the total radioactivity added 32% of this eictivity is associated with the cellular lipid fraction and 64% with the nonlipid (presumably protein) fraction. Of the lipid-associated activity, the distribution of I into cellular phospholipids, lysophosphatides, triacylglycerides, and free fatty acids is shown in Table IVB. The iodination reaction labels a variety of lipids of all classes without preference to charge, acid-base properties, or number of fatty acyl constituents, appears not to label intracellular lipids (e.g., cardiolipin, a major mitochondrial component), and labels lipids on both the exterior and interior hydrophilic surfaces of the plasma membrane (i.e., phosphatidylcholine and sphingomyelin as well as phosphatidyleth-anolamine and phosphatidylserine) (Table IVB). ... 

In addition to thyroglobulin, other iodinated proteins have been found in the thyroid. Their importance in pathology must not be underestimated. Some iodinated proteins with mobility characteristics identical to those of iodoalbumin escape from the thyroid and are found in the plasma in some pathological conditions. Another iodoprotein is associated with cell particles, and its concentration increases in thyroid tumors. 

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