A goitre is a swelling in the neck due to an enlarged thyroid gland. The most common cause for goitre in the world is iodine deficiency.
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.
There are fears by some health workers that a resurgence of goitre might occur because of the trend to use rock salt (which has not been fortified with iodine) and also less salt use in general.