Class: Radiopharmaceutical (β- and γ-emitter).
Mechanism:
- Thyroid follicular cells actively uptake iodine via the sodium-iodide symporter (NIS).
- I-131 delivers localized β-radiation → destroys thyroid tissue or residual/ metastatic differentiated thyroid cancer cells.
- The γ-emission allows post-treatment imaging.
Clinical Indications
- Differentiated thyroid cancer (DTC)
- Papillary and follicular thyroid carcinoma (not medullary or anaplastic).
- Ablation of thyroid remnant after surgery.
- Treatment of recurrent or metastatic disease.
- Benign thyroid disease
- Hyperthyroidism from Graves’ disease, toxic multinodular goiter, or toxic adenoma.
Administration / Preparation
- Oral capsule or liquid (I-131).
- Pretreatment preparation:
- Thyroid hormone withdrawal (levothyroxine stopped ~2–4 weeks) OR recombinant TSH (thyrotropin alfa, Thyrogen®) to increase TSH and stimulate iodine uptake.
- Low-iodine diet for 1–2 weeks before therapy.
- Dosing (varies by indication, patient size, institutional protocols):
- Ablation after thyroidectomy: ~30–100 mCi.
- Metastatic disease: 100–200 mCi.
- Hyperthyroidism: 5–15 mCi (smaller doses).
Adverse Effects
- Short-term:
- Nausea, sialadenitis (salivary gland inflammation), taste changes, dry mouth.
- Long-term:
- Hypothyroidism (almost universal in hyperthyroidism treatment).
- Secondary malignancies (rare, leukemia or solid tumors at high cumulative doses).
- Infertility (dose-dependent, more concern in younger patients).
- Pulmonary fibrosis (if lung mets + high dose).
Precautions / Counseling
- Radiation safety: avoid close contact with children and pregnant women for several days after dose; follow strict hygiene rules.
- Contraindicated in pregnancy and breastfeeding.
- Encourage hydration and sour candies to stimulate salivary flow (reduce sialadenitis).
Pharmacist Pearls
- RAI is not effective in medullary or anaplastic thyroid cancers (these do not uptake iodine).
- Always confirm pregnancy status before administration.
- Check if patient is on amiodarone or iodinated contrast (can block uptake).
- Long-term thyroid hormone replacement (levothyroxine) is usually needed.