[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Antithyroid agents](https://durgical.com/en/pharmacology/classes/antithyroid-agents/) › Sodium iodide (131I)

[Pharmacology](https://durgical.com/en/pharmacology/) · [Antithyroid agents](https://durgical.com/en/pharmacology/classes/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

- Class: [Antithyroid agents](https://durgical.com/en/pharmacology/classes/antithyroid-agents/)
- Transporter: Sodium-iodide symporter (NIS) of the thyroid
- ATC code: V10XA01

Also called: Radioactive iodine, Radioiodine, Iodine-131

⇄**Sodium-iodide symporter (NIS) of the thyroid**Sodium iodide (131I)

ⓘ 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.

**Target in 3D:** Sodium-iodide symporter (NIS) of the thyroid. PDB structure 7UV0 (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/7UV0) · Drag to rotate · pinch to zoom

## 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) →](https://durgical.com/en/conditions/graves-disease/)
- Selected thyroid cancers that take up iodine, after surgery [Thyroid cancer (papillary) →](https://durgical.com/en/conditions/thyroid-cancer/)

## Adverse effects

### Common

- Neck swelling or tenderness
- Salivary gland inflammation and dry mouth
- Nausea and «radiation sickness»
- Tear gland problems and dry eyes

### Serious (rare, important)

- Hypothyroidism, often permanent
- Radiation thyroiditis that can worsen hyperthyroidism or trigger thyroid storm
- Bone marrow depression with high cancer doses
- Severe, irreversible hypothyroidism in the newborn if given in pregnancy

## 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.

## Memory aid

131I = a Trojan horse: the thyroid's own iodide pump carries it in, then it explodes with beta radiation.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/sodium-iodide/)

## Related drugs

- [Potassium iodide](https://durgical.com/en/pharmacology/potassium-iodide/)
- [Methimazole](https://durgical.com/en/pharmacology/methimazole/)
- [Levothyroxine](https://durgical.com/en/pharmacology/levothyroxine/)

## Related conditions

- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Thyroid cancer (papillary)](https://durgical.com/en/conditions/thyroid-cancer/)

[← Back to the class: Antithyroid agents](https://durgical.com/en/pharmacology/classes/antithyroid-agents/)

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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=380e8e26-0625-4233-b36e-afb9f66e8a77) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=V10XA01) (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/pharmacology/sodium-iodide/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/sodium-iodide/