Drug reference

iodine (131i)

Thyroid inhibitor, Radioisotope · Antithyroid, Diagnostic agent

Thyroid inhibitor, RadioisotopeAntithyroid, Diagnostic agent

CDSCO approved

EXCRETION

—

not curated

INTERACTIONS

—

none in our sources

PREGNANCY

—

not curated

Mechanism

Radioactive iodine (131I) is concentrated by the thyroid and incorporated into colloid, where it emits radiation (beta particles). The beta particles penetrate only 0.5–2 mm of tissue, causing pyknosis and necrosis of thyroid follicular cells, followed by fibrosis.

Indications

Hyperthyroidism (Graves’ disease, toxic nodular goiter - treatment of choice after 25 years of age or if contraindication to surgery)Metastatic carcinoma of thyroid (palliative therapy after thyroidectomy for papillary or follicular types)

Dosing

Adult: Diagnostic: 25–100 µ curie. Therapeutic: Average 3–6 m curie (calculated based on tracer studies and thyroid size); higher doses for toxic multinodular goiter. For metastatic carcinoma, higher doses are required and prior stimulation with TSH is recommended.

Pharmacokinetics

Half-life

Physical half-life 8 days.

Contraindications

pregnancy (foetal thyroid destroyed, cretinism, other abnormalities if given during first trimester)
young patients (increased risk of hypothyroidism later, fear of genetic damage/cancer)