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# Potassium iodide

Stable iodide that floods the thyroid. It protects the gland in a radiation emergency and briefly slows hormone release in severe hyperthyroidism.

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: V03AB21
- WHO Model List of Essential Medicines

Also called: KI

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

ⓘ The figure shows iodide bound in the sodium-iodide symporter of the rat thyroid, very similar to the human transporter; a large load of stable iodide competes with radioactive iodine for this transporter.

**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

A large amount of iodide saturates the thyroid. It competes with radioactive iodine for uptake, so in a nuclear emergency the gland takes in less radioactivity. In hyperthyroidism, high intracellular iodide inhibits organification and, more importantly, the release of stored hormone, so symptoms improve within days. It also shrinks the gland and reduces its blood flow, which helps before surgery. The block is temporary: the gland can escape after some weeks.

- Excess iodide saturates the thyroid's iodide transporter
- Less radioactive iodine is taken up
- High iodide blocks organification and hormone release
- The gland shrinks and becomes less vascular

## Pharmacokinetics

**Absorption**
Rapidly absorbed by mouth (tablets or oral solution)

**Distribution**
Concentrated in the thyroid; crosses the placenta and reaches breast milk

**Elimination**
Mainly in the urine

**Onset and duration**
Hormone release falls within days; the effect fades after some weeks

## Indications

- Thyroid protection in a nuclear radiation emergency, when public health officials advise it
- Thyroid storm, given after a thionamide has been started [Graves disease (hyperthyroidism) →](https://durgical.com/en/conditions/graves-disease/)
- Preparation of the hyperthyroid gland for surgery

## Adverse effects

### Common

- Metallic taste
- Swelling of the salivary glands
- Nausea, vomiting, diarrhoea and stomach ache
- Rash

### Serious (rare, important)

- Allergic reactions, with fever, joint pain, swelling or difficulty breathing
- Iodine-induced hyperthyroidism or hypothyroidism in susceptible thyroids
- Worse thyrotoxicosis if the gland escapes the block

## Contraindications

- Allergy to iodine
- Dermatitis herpetiformis
- Hypocomplementaemic vasculitis
- Nodular thyroid disease together with heart disease

## Key interactions

**Thionamides (methimazole, propylthiouracil)**
Iodide is given after the thionamide is started; given first, it can fuel more hormone synthesis

**Radioactive iodine (131I) treatment or scans**
Stable iodide blocks uptake and delays or spoils 131I therapy

## What is monitored

- Thyroid function
- Heart rate and symptoms in hyperthyroidism
- Signs of allergy or iodism

## Administration and safety

- In a radiation emergency it is taken only when public health officials advise it, and only for the time they say
- The amount differs by age group, as printed on the package; babies under one month who receive it have thyroid function checked
- Not the same as iodine-povidone antiseptics or Lugol solution: confirm the product

## Nursing care

- Ask about iodine allergy, dermatitis herpetiformis and thyroid or heart disease before giving it
- In thyroid storm, confirm that the thionamide was given first
- Watch for rash, fever, joint pain or salivary swelling

## Pregnancy and breastfeeding

Chronic use in pregnancy is avoided because iodide crosses the placenta and can cause fetal goitre; a short course in a radiation emergency is a separate decision.

## Memory aid

KI = «Keep Iodine» out: floods the gland so radioactive iodine and hormone release are both shut out, but only for a while.

## Test yourself

Think of the answer, then tap the question to see it.

## How does potassium iodide protect the thyroid in a nuclear accident?

Stable iodide saturates the gland and competes for uptake, so much less radioactive iodine is concentrated there.

## Why is iodide given after, not before, a thionamide in thyroid storm?

Given first, iodide supplies substrate for new hormone synthesis; once synthesis is blocked, iodide mainly stops the release of stored hormone.

## Why is iodide given before surgery for Graves disease?

It shrinks the hyperplastic gland and reduces its blood flow and fragility, making surgery easier and safer.

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## Related drugs

- [Propylthiouracil](https://durgical.com/en/pharmacology/propylthiouracil/)
- [Methimazole](https://durgical.com/en/pharmacology/methimazole/)
- [Sodium iodide (131I)](https://durgical.com/en/pharmacology/sodium-iodide/)

## Related conditions

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

[← 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: [Potassium Iodide: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a611043.html) (MedlinePlus (U.S. National Library of Medicine), 2016); [DailyMed - POTASSIUM IODIDE solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d9e02a48-8129-9076-78b2-db01750bd220) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=V03AB21) (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.

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