Radioactive iodine (RAI, I‑131) dosing varies by indication. For hyperthyroidism doses typically range 5–30 mCi (often 10–15 mCi). Toxic nodular goiter commonly uses 30–150 mCi. Thyroid remnant ablation or adjuvant therapy is generally 30–150 mCi, while treatment for metastatic disease often involves higher courses of 100–200+ mCi. Exact mCi is individualized by disease, gland size and uptake.

After the Chernobyl accident people took stable nonradioactive iodine to saturate the thyroid and block uptake of radioactive iodine isotopes such as I‑131. Early administration — ideally before or shortly after exposure — sharply reduces thyroid radiation dose and lowers the risk of thyroid cancer, especially in children. Potassium iodide is effective only for short-term, specific exposures and must be dosed correctly.

Radioactive iodine (I-131) therapy treats differentiated thyroid cancers by exploiting thyroid cells’ iodine uptake to ablate residual thyroid tissue and iodine‑avid metastases after surgery. Doses are individualized; side effects include dry mouth, altered taste, salivary gland inflammation, nausea and long‑term hypothyroidism. It’s contraindicated in pregnancy and breastfeeding and requires radiation precautions; not effective for most non‑thyroid cancers.

A normal radioactive iodine scan shows a thyroid of normal size and shape with even, symmetric uptake across both lobes and no focal “cold” or “hot” nodules. Background activity is low. Quantitative uptake falls within the lab reference range (commonly about 10–30% at 24 hours). Findings indicate normally functioning thyroid tissue without focal autonomy or significant inflammation.

Common side effects after a 50 mCi radioactive iodine dose include nausea, neck pain or swelling, sore throat, dry mouth, altered taste, salivary gland pain or swelling, fatigue, and transient worsening of hyperthyroid symptoms. Longer-term effects can include permanent hypothyroidism needing lifelong replacement, temporary reduced fertility, and a very small increased risk of secondary cancers. It is contraindicated in pregnancy and breastfeeding.

MCI stands for methylchloroisothiazolinone, a synthetic isothiazolinone preservative often used with methylisothiazolinone (MI) in cosmetics, shampoos, and household products to prevent microbial growth. It’s associated with allergic contact dermatitis in sensitive individuals, so products may list MCI/MI or the full chemical name on labels, and some formulations now avoid or limit its use.