A goitre (or goiter) (Latin struma) is a swelling in the neck due to an enlarged thyroid gland. The most common cause for goitre in the world is iodine deficiency. Other causes are:

• Hashimoto's thyroiditis
• Graves-Basedow disease
• juvenile goitre
• neoplasm of the thyroid
• thyroiditis (acute, chronic)
• side-effects of pharmacological therapy

Iodine is necessary for the synthesis of the thyroid hormones, triiodothyronine and thyroxine (T3 and T4). When iodine is not available these hormones cannot be made. In response to low thyroid hormones, the pituitary gland releases thyroid stimulating hormone (TSH).

Thyroid stimulating hormone acts to try and increase synthesis of T3 and T4, but also causes the thyroid gland to grow in size as a type of compensation.

Goitre was previously common in many areas that were deficient in iodine in the soil. The condition now is practically absent in affluent nations, where table salt is supplemented with iodine.

There are fears by some health workers that a resurgence of goitre might occur because of the trend to use rock salt and/or sea salt (which has not been fortified with iodine) and also less salt use in general.