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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# Iodine

Iodine is the raw material of thyroid hormones; its lack causes goitre, hypothyroidism and, in pregnancy, brain damage in the baby.

Updated 2026-11-01

## Key facts

- T4 has four iodine atoms and T3 three; T3 is the active form in tissues.
- Steps in the thyroid: uptake by the sodium-iodide symporter (NIS), oxidation and binding to thyroglobulin by thyroid peroxidase (TPO), coupling and release.
- Iodine deficiency is the world's most important preventable cause of intellectual disability; iodised salt is the preventive measure.
- In Colombia salt iodisation has been mandatory since the mid-twentieth century and endemic goitre fell sharply.

## In detail

- Sources: iodised salt, fish and shellfish, seaweed, dairy, eggs. Mountain soils and soils far from the sea tend to be poor in iodine.
- Uptake: NIS in the thyrocyte basal membrane concentrates iodide; pendrin passes it into the colloid.
- Organification: TPO, with hydrogen peroxide, oxidises iodide and binds it to tyrosines of thyroglobulin (MIT, DIT); it then couples MIT + DIT = T3 and DIT + DIT = T4.
- Release and activation: thyroglobulin is taken back up and digested in lysosomes; deiodinases (selenium enzymes) convert T4 to T3 in tissues.
- Deficiency: goitre (high TSH makes the gland grow), hypothyroidism, cretinism if the mother is deficient in pregnancy, more miscarriage and perinatal death.
- Excess: in a healthy thyroid it briefly blocks synthesis (Wolff-Chaikoff effect); in nodular goitres it can cause hyperthyroidism (Jod-Basedow). Amiodarone and iodinated contrast are common sources.
- Lab: urinary iodine (measures the status of a population, not of one person); TSH and free T4 for thyroid function.

**Also called**
I, Iodide

## Clinical notes

- Newborn TSH screening finds congenital hypothyroidism, whose commonest cause worldwide is iodine deficiency and, where salt is iodised, thyroid dysgenesis.
- Potassium iodide in a nuclear emergency saturates the thyroid and reduces uptake of radioactive iodine.
- Radioactive iodine (I-131) is taken up by NIS and used to treat hyperthyroidism and differentiated thyroid cancer.

## Related conditions

- [Goitre](https://durgical.com/en/conditions/goitre/)
- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Thyroid nodule](https://durgical.com/en/conditions/thyroid-nodule/)

## Related

- [Selenium](https://durgical.com/en/biochemistry/topics/selenium/)

## Test yourself

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

## Which enzyme binds iodine to thyroglobulin?

Thyroid peroxidase (TPO), which oxidises iodide with hydrogen peroxide and binds it to tyrosines; methimazole and propylthiouracil block it.

## Why does iodine deficiency cause goitre?

Little hormone is made, TSH rises and stimulates the thyroid to grow.

## What is the Wolff-Chaikoff effect?

A temporary block of thyroid hormone synthesis after a large iodine load; a healthy thyroid escapes within days.

## How is the iodine status of a population measured?

With the median urinary iodine of groups such as schoolchildren or pregnant women; a single sample says little about one person.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Iodine in diet: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002421.htm) (MedlinePlus, US National Library of Medicine, 2026); [Iodine deficiency](https://www.who.int/data/nutrition/nlis/info/iodine-deficiency) (World Health Organization, 2026); [Congenital hypothyroidism: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/congenital-hypothyroidism/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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