Goiter is an enlarged thyroid gland, and it has several causes. A biosynthesis defect in one of the enzymes that build thyroid hormone, or a shortage of iodine, the raw material for it, leaves the gland making too little hormone, so the pituitary raises thyroid-stimulating hormone (TSH), the hormone that drives the gland, and the sustained drive makes the gland grow. Autoimmune disease enlarges the gland in two different ways: thyroid-stimulating immunoglobulins (TSI) in Graves’ disease stimulate the gland directly, while in Hashimoto’s thyroiditis the fall in hormone production raises TSH. Nodular change is the fourth route, in which discrete lumps rather than uniform tissue enlarge the gland. Goiter is more common in women than in men, probably because autoimmune thyroid disease is more common in women and because the demand for iodine rises during pregnancy.
Diffuse non-toxic goiter refers to the enlargement of the thyroid gland without the presence of nodules and hyperthyroidism. It is also called simple goiter or colloid goiter.
The most common cause is iodine deficiency. Where more than 5% of the population is affected the enlargement is referred to as endemic goiter, and where iodine deficiency is not endemic the same diffuse enlargement is called sporadic goiter, whose cause is mostly unknown. Endemic goiter may also be due to the use of goitrogen-containing foods such as cruciferae plants, cassava root, and milk in areas with grasses containing goitrogen material; in some areas the food supply and lifestyle can therefore endemically cause iodine deficiency.
In an endemic area of iodine deficiency the goiter is a compensatory mechanism for the lower synthesis of thyroid hormones — but it is surprising that in iodine-deficient goiter the level of TSH is normal or slightly increased, which suggests increased sensitivity to TSH. Because the deficiency is the cause, endemic goiter is prevented at population level by iodising salt rather than by treating each person individually.
Uniform enlargement is not the only pattern, because over the years the same gland can develop discrete nodules instead.