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.

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.

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.

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.

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

Sources: Iodine in diet: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Iodine deficiency (World Health Organization, 2026); Congenital hypothyroidism: MedlinePlus Genetics (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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Representative model, for educational purposes only.