What are the two morphological phases of nontoxic diffuse goiter?
The hyperplastic phase and the phase of colloid involution. In the hyperplastic phase, the thyroid gland is diffusely and symmetrically enlarged, the follicles are lined by crowded columnar cells, which may form projections similar to those seen in Graves disease. The accumulation is not uniform. If dietary iodine subsequently increases or if the demand for thyroid hormone decreases, the stimulated follicular epithelium involutes to form an enlarged, colloid-rich gland (colloid goiter). The follicular epithelium is flattened and cuboidal, and colloid is abundant during periods of involution.