Colloid goiter/simple goiter/diffuse nontoxic goiter.

Gross: The thyroid gland is enlarged. cut surface is brown and translucent.
Microscopic:  Flattened follicular epithelial lining ( normally   the lining is low cuboidal to columnar). The follicles and enlarged and filled with abundant colloid.
Note that the above finding is seen in phase of colloid involution.
In the hyperplastic phase, there will he hyperplasia of the lining epithelum. The crowded columnar cells sometimes may project into the follicular lumen.