Pituitary adenomas may
or may not secrete hormones. The majority of tumors do secrete hormones
and can be classified by which hormone(s) they are producing.
The most common type of pituitary tumor is the prolactin-secreting
prolactinoma. These account for nearly 30% of all pituitary tumors.
Lactation is the primary symptom of this tumor. Lactation can occur
in other types of tumors as well.
The second most prevalent type, comprising approximately 25% of
pituitary tumors, is called a null cell adenoma. Null cell adenomas
are considered non-functioning because they do not produce any hormone.
They are sometimes referred to as non-secreting tumors. Lactation
can be a symptom that occurs with this tumor type even though it
is not a prolactinoma.
Growth hormone-secreting tumors, seen in about 10-15% of patients,
are associated with acromegaly, a clinical syndrome that involves
a thickening of the bones of the hands, feet, cheeks, and jaw.
Another 10-15% of pituitary adenomas are accounted for by ACTH-secreting
tumors, which are often the cause of Cushing's disease.
Some pituitary tumors are also capable of secreting more than one
hormone. The most common multi-hormonal tumor secretes both growth
hormone and prolactin.
Tumors that secrete TSH or FSH and LH are not frequently seen.