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A radioisotope of iodine that emits beta and gamma rays, has a half-life of 8.05 days, and is used as a tracer in thyroid studies and as therapy in hyperthyroidism and thyroid cancer.


(i'o-din?, -den?) [Gr. ioeides, violet-colored + -ine] I
A nonmetallic chemical element of the halogen group, atomic weight (mass) 126.904, atomic number 53, specific gravity (solid, 20°C) 4.93. It is a black crystalline substance with a melting point of 113.5°C; it boils at 184.4°C, giving off a characteristic violet vapor. Sources of iodine include vegetables, esp. those growing near the seacoast; iodized salt; and seafoods, esp. liver of halibut and cod, or fish liver oils.
CAS # 7553-56-2


Iodine is part of the hormones triiodothyronine (T3) and thyroxine (T4), and prevents goiter by enabling the thyroid gland to function normally. The amount of iodine in the entire body averages 50 mg, of which 10 to 15 mg is found in the thyroid. The adult daily requirement for iodine is from 100 to 150 µg. Growing children, adolescents, pregnant women, and those under emotional strain need more than this amount of iodine.

Deficiency Symptoms

Iodine deficiency in the diet may lead to simple goiter characterized by thyroid enlargement and hypothyroidism. In young children, this deficiency may result in retardation of physical, sexual, and mental development, a condition called cretinism.


131I See: radioactive iodine

protein-bound iodine

Abbreviation: PBI
Iodine that is attached to serum protein. In the past, thyroid function was tested with a serum measurement of PBI.

radioactive iodine

Any of the radioactive isotopes of iodine, esp. iodine-131, used in diagnosis and treatment of thyroid disorders and in the treatment of toxic goiter and thyroid carcinoma.
See: radioiodine

tincture of iodine

See: tincture


a chemical element, atomic number 53, atomic weight 126.904, symbol I. See Table 6. Iodine is essential in nutrition, being especially prevalent in the colloid of the thyroid gland. It is used in the treatment of hypothyroidism and as a topical antiseptic. Iodine is a frequent cause of poisoning. See also iodism.

a radioisotope of iodine having a half-life of 60 days and a principal gamma-ray photon energy of 28 keV; used as a label in radioimmunoassays and other in vitro tests, and also for thyroid imaging. Symbol 125I.
a radioisotope which concentrates in chromaffin cells; used in diagnostic scintigraphy, e.g. in cases of pheochromocytoma.
a radioisotope of iodine having a half-life of 8.1 days and a principal gamma-ray photon energy of 364 keV; used in treatment of hyperthyroidism and carcinoma of the thyroid, in thyroid function testing, and in imaging of the thyroid gland and other organs. Symbol 131I.
iodine deficiency
may occur in all species under certain conditions; in dogs and cats, a factor in all-meat diets. See also goiter.
iodine contrast agents
iodine salts are opaque to x-rays; therefore they can be combined with other compounds and used as contrast media in diagnostic x-ray examinations.
iodine nutritional deficiency
is characterized by goiter, neonatal mortality and alopecia.
iodine poisoning
occurs usually due to accidental overdosing. It causes lacrimation, anorexia, coughing due to bronchopneumonia, and a heavy dandruff. Paradoxically, iodine excess may result in thyroid hyperplasia and goiter, especially in the young.
protein-bound iodine
a test of thyroid function. See also protein-bound iodine (PBI) test.
radioactive iodine
see iodine-125, iodine-131 (above).
iodine residues in milk
careless use of iodine-based teat dips results in unacceptable residues of iodine in milk.
iodine solution
contains 2% free iodine and 2.4% sodium iodide in an aqueous solution.
iodine solution (strong)
contains 5% free iodine and 10% potassium iodide in an aqueous solution.
tamed iodine
iodine trapping
the selective absorption of iodine from the circulation by the thyroid gland.