Pharmacology · Antithyroid agents
Sodium iodide (131I) !
Radioactive iodine swallowed as a capsule or liquid. The thyroid takes it up and its radiation destroys overactive or cancerous thyroid tissue.
Updated 2026-11-01
Also called: Radioactive iodine, Radioiodine, Iodine-131
The figure shows iodide bound in the sodium-iodide symporter of the rat thyroid, very similar to the human transporter that brings 131I into the cells.
Mechanism of action
Thyroid follicular cells take up 131I through the sodium-iodide symporter and concentrate it far above plasma levels; thyroid peroxidase then incorporates it into thyroglobulin. 131I emits beta particles that travel only a short distance in tissue, so the radiation damages the cells that hold it. Over the following weeks to months the gland shrinks and makes less hormone; hypothyroidism is a common end result.
- The sodium-iodide symporter carries 131I into thyroid cells
- 131I is trapped and built into thyroglobulin
- Short-range beta radiation destroys the thyroid cells
- Over weeks to months, hormone production falls
Pharmacokinetics
- Absorption
- Swallowed as capsules or oral solution and rapidly absorbed
- Distribution
- Concentrated in thyroid tissue; also reaches salivary glands, stomach and breast
- Elimination
- Unabsorbed iodide leaves mainly in the urine, which is radioactive for some days
- Onset and duration
- The effect develops over weeks to months
Indications
- Hyperthyroidism, such as Graves disease or toxic nodular goitre Graves disease (hyperthyroidism) →
- Selected thyroid cancers that take up iodine, after surgery Thyroid cancer (papillary) →
Adverse effects
Common
- Neck swelling or tenderness
- Salivary gland inflammation and dry mouth
- Nausea and «radiation sickness»
- Tear gland problems and dry eyes
Contraindications
- Pregnancy
- Breastfeeding
- Vomiting and diarrhoea
- Thyroid cancers that do not take up iodide
- Concurrent antithyroid drug treatment
Key interactions
- Thionamides (methimazole, propylthiouracil)
- They reduce the effect of 131I; they are not given at the same time
- Stable iodide (potassium iodide, iodinated contrast, amiodarone)
- Saturates the gland and blocks 131I uptake
What is monitored
- Pregnancy test before treatment
- Free T4 and TSH regularly afterwards to detect hypothyroidism
- Blood counts after high cancer doses
Administration and safety
- Given by nuclear medicine staff under radiation safety rules; it is not handled like an ordinary oral drug
- Pregnancy is ruled out before administration
- Urine, saliva and sweat are radioactive for some days; the patient receives written instructions on contact and hygiene
Nursing care
- Confirm a negative pregnancy test and that the patient is not breastfeeding
- Follow the radiation safety plan: time, distance and handling of body fluids
- Encourage fluids and frequent voiding, as instructed, to clear unused 131I
- Teach the signs of hypothyroidism and the need for lifelong thyroid tests
Pregnancy and breastfeeding
Contraindicated: it crosses the placenta and destroys the fetal thyroid, causing severe and irreversible hypothyroidism in the newborn.
Test yourself
Think of the answer, then tap the question to see it.
Why does 131I destroy thyroid tissue while sparing most of the body?
The thyroid concentrates iodide through the sodium-iodide symporter, and 131I's beta particles travel only a short distance in tissue.
What is the most common long-term result of radioactive iodine for Graves disease?
Hypothyroidism, which needs lifelong levothyroxine; free T4 and TSH are checked regularly.
Why is 131I contraindicated in pregnancy?
It crosses the placenta and the fetal thyroid takes it up, causing severe, irreversible neonatal hypothyroidism.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - SODIUM IODIDE I-131 kit (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).
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Representative model, for educational purposes only.