Plasma organic iodine

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. 

A low or even a moderate single dose of iodide causes no or only a minor, reduction of hormone production in a normal thyroid. An excessive single dose of iodide leads, via inhibition of the thyroid peroxidase (TPO) mediated iodination (the Wolff-ChaikofF effect), to a transient decrease of intrathyroidal hormone concentration. This was first demonstrated by Wolff and Chaikoff (1949). When high plasma levels of iodide are sustained by repeated iodine administration, the inhibiting effect disappears and normal organic iodine levels in the thyroid are restored, with a decrease in thyroid sodium symporter, as demonstrated by Eng et al. (1999). This effect was demonstrated by Haydl and Waldhausl (1975) in the course of health resort treatments administering iodine-containing mineral waters. 

Higher urinary iodine No change in plasma inorganic iodine Increased thyroid iodine clearance Increased absolute iodine uptake by the thyroid Increased thyroid organic pool of iodine... 

The thyroid releases predominantly thyroxine (T4). However, the active form appears to be triiodothyronine (T3) T4 is converted in part to T3, receptor affinity in target organs being 10-fold higher for T3. The effect of T3 develops more rapidly and has a shorter duration than does that of T4. Plasma elimination tip for T4 is about 7 d that for T3, however, is only 1.5 d. Conversion of T4 to T3 releases iodide 150 pg T4 contains 100 pg of iodine. 

Extraction of iodine contained in organic (humic and fulvic) components of the soils was performed by shaking 2g soil samples in 20 ml of 5% TMAH, using a table shaker, for 4h. The mixture was then centrifuged, and the supernatant analyzed using ion chromatography and inductively coupled plasma mass spectrometry (ICP-MS). Using the NIST SRM soils with well-known total iodine contents, we also evaluated extraction variables, such as the temperature (TMAH extraction under either room temperature or 80°C), on quantitative iodine extraction. 

Figure 15.3 Simultaneous ECD and ICP-MS chromatograms of a seawater sample for fhe deferminafion of brominafed and iodinafed volafile organic compounds. Reproduced from Schwarz and Neumann (2002). The top figure shows fhe chromafograms of brominafed and iodinated volafile organic compounds in seawater measured by ECD, the bottom figure shows fhe chromafogram measured by inductively coupled plasma mass spectrometry (ICP-MS).

As soon as radioactive tracers of iodine, especially iodide, thyroxine (T4), and triiodothyronine (T3), became available after the end of World War II, research began on the kinetics of radioiodine in humans, as well as in experimental animals. As measuring devices improved, radioactivity could be measured not only in the blood plasma, but also in other body fluids, and external measurement of the radioactivity in the thyroid and in other organs of the body soon became feasible. A rich body of experimental data ensued there was a need to synthesize these data into a unifying theoretical structure. 

Recent work on the three main functions of the thyroid gland— the collection of iodide from the plasma, the transformation of iodide into organically bound iodine and the release of the hormone into the circulation—has been reviewed. 

In discussing iodide metabolism, we need only be concerned with the fate of inorganic iodides because organic iodides (tyrosine derivatives) are absorbed and enter the normal plasma pool of organic iodides produced by the thyroid. The site and the mechanism of absorption of organic iodides in the intestine are not exactly known, but somehow the dietary iodine is transported from the intestinal lumen into the blood. 

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