Goiter is enlargement of the thyroid gland due to any cause. There are two major classifications of goiter: toxic and non-toxic. The case being studied here is a non-toxic goiter. There are two major morphological categories of non-toxic goiter: diffuse and multinodular. These different morphological forms represent the same disease at different points in time. The diffuse goiter arises first as an appropriate reaction of the body to low levels of thyroid hormones. Low levels of these hormones cause the body to secrete more TSH which results in the hyperplasia of the follicular cells of the thyroid. This results in a diffuse non-colloid goiter. Over time these follicular cells produce large amounts of colloid in response to the stimulation from TSH. This results in a diffuse colloid goiter.

Colloid is defined as the " material" in the follicle which can be any combination of the following:

Thyroglobulin, iodinated thyroglobulin, and the enzymes responsible for the iodination reaction. In this slide note the large colloid filled follicles. In mutlinodular goiter, the follicles would be heterogeneous in their composition. Some of the follicles would be colloid filled, some would be small and fibrotic, some would be hemorrhagic and others would be cystic.