Goitre is a condition characterized by the enlargement of the thyroid gland. It is primarily caused by a deficiency of iodine in the diet. The thyroid gland requires iodine to produce thyroid hormones which are essential for regulating various processes in the body. When there is a lack of iodine, the thyroid gland enlarges in an attempt to compensate for the low levels of iodine. This enlargement leads to the formation of a goitre.

Options 2, 3, and 4 - Chlorine, sodium, and calcium deficiencies are not known to cause goitre. While these minerals are important for various bodily functions, their deficiencies do not directly affect the thyroid gland. Therefore, option 1, iodine deficiency, is the correct answer.