It may be difficult to differentiate pituitary hyperplasia from an adenoma. The reticulin stain can often be used to help this differentiation [18]. In hyperplasia, the normal acinar structure of the pituitary gland is maintained but it is enlarged, whereas adenomas typically demonstrate disruption of the reticular acinar structure.

Histopathology may demonstrate diffuse or nodular hyperplasia, typically identifiable with a silver/reticulin stain [6]. In diffuse hyperplasia, an increase in the size of glandular cells and acini is seen throughout the gland, without disruption of the acinar network [18] (Fig. 9.4). Nodular hyperplasia appears as a local, nodular growth of pituitary cells, with enlargement and possible disruption of the acinar network. Thyrotroph hyperplasia is diffuse in 69 % of cases and nodular in 25 % [10]. Lactotroph hyperplasia is generally of the diffuse type [6].

When clinical and imaging characteristics lead to suspicion of pituitary hyperplasia, the underlying hormonal cause should be sought. If the inciting cause is identified and treated, pituitary hyperplasia is typically reversible. For example, thyrotroph hyperplasia is generally reversible following treatment with thyroid hormone replacement [9]. Lactotroph hyperplasia is partially reversible following pregnancy or lactation. Nulliparous women have less massive pituitary glands than multiparous women [19].

In the rare cases when mass effect results in severe headache, visual loss, or panhypopituitarism, surgical intervention for tissue diagnosis and decompression may be indicated.