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CLINICAL PHARMACOLOGY

Pharmacokinetics

Absorption

Following oral administration, Sodium Iodide I-131 is readily absorbed from the gastrointestinal tract.

Distribution

Following absorption, the iodide is primarily distributed within the extra-cellular fluid of the body. It is trapped by the thyroid. The thyroid uptake of iodide is usually increased in hyperthyroidism and in goiter with impaired hormone synthesis, decreased in hypothyroidism, and normal to decreased in hypothyroidism receiving iodine. It should be noted that the uptake of radioactive iodide is a function of stable iodide concentration in the serum and the functional state of the thyroid. The iodine concentrating mechanism of the thyroid, termed the iodide trap or pump, accounts for an iodide concentration of some 25 times plasma levels, but may increase as much as 500 times under certain conditions. It is also concentrated by the stomach, choroid plexus, and salivary glands, but is not protein-bound.

Metabolism

Trapped iodide is oxidized to iodine and organically incorporated so rapidly that the iodide trap of the thyroid contains less than 0.2 % free iodide in comparison to the organically bound iodine. This process results in further concentration of iodine in the thyroid gland to about 500 times that in the blood.

The iodinated organic compounds chiefly consist of thyroxine (T4) and triiodothyronine (T3), which are bound by thyroglobulin in the follicular colloid. T4 and T3 are released by enzymatic proteolysis of thyroglobulin into the blood where they are specifically bound and transported by plasma thyroid binding proteins. These reactions are primarily under the control of anterior pituitary gland release of thyroid stimulating hormone (TSH) and hypothalamic thyroid release factor (TRF).

Excretion

Sodium Iodide I-131 is excreted by the kidneys. The normal range of urinary excretion is 37 to 75 % of the administered dose, varying with the thyroid and renal function of the patient.

Pharmacodynamics

Stimulation of radioiodide uptake may be achieved by the administration of thyrotropin or placing the patient on a low iodine diet prior to treatment. Palliative effects may be seen in patients with papillary and/or follicular carcinoma of the thyroid. Radioiodide will not be taken up by giant cell and spindle cell carcinoma of the thyroid or by amyloid solid carcinomas.

Last reviewed on RxList: 10/6/2011
This monograph has been modified to include the generic and brand name in many instances.

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